Happens once in All Quiet on the Western Front, when a newbie in the trenches is getting hysterical to the point of trying to leave the bomb shelter. Everybody else in the shelter beats him up until he doesn't try to leave any more. The narrator tells us that it's not pleasant, but it's the only thing that helps.
God often sends His angels to give words of encouragement in The Bible, but on the top of the list of epic "Get A Hold of Yourself"s from God Himself is when Elijah has a Heroic BSoD. Full text here, but here's a sample:
"Then a great and powerful wind tore the mountains apart and shattered the rocks before the LORD, but the LORD was not in the wind. After the wind there was an earthquake, but the LORD was not in the earthquake. After the earthquake came a fire, but the LORD was not in the fire. And after the fire came a gentle whisper."
In The Brotherhood of the Conch novel Shadowland, Anand and Nisha visit the magicians' hideout and are immediately hit by a charm that causes exhaustion and despair. Anand, who is much more strongly affected than Nisha, falls to his knees. Nisha shakes his shoulder, then slaps him in the face, which weakens the charm's effects enough that he can keep walking.
In Brothers of the Snake, Priad has to smack his battle-brother Natus across the head to get his attention off the blasphemous text the heretics defaced the statue of the Emperor with.
In The Pool of the Black One, Conan shakes Sancha until she says she will help him rescue the prisoners; then he gives her an encouraging (and heavy) slap on the back.
Contraband Rocket, a 1956 sci-fi novel by Lee Correy, involves some rocket enthusiasts restoring and launching a Retro Rocket. It's pointed out during the launch briefing that one man panicking during blast-off can kill them all, so there's a billy club under the couch pads of each seat. Apparently one gets used, because an officer reports a crewman with a busted jaw.
In Dave Barry Does Japan, the author receives a big SLAP (possibly from himself) when he goes off on a long rant about American bureaucracy.
Nanny Ogg delivers a much-needed slap to Granny Weatherwax in Wyrd Sisters, less to calm her down and more to focus her rage. It's so powerful that it lifts both of them off their feet.
Used repeatedly in Making Money to interrupt Hubert Turvey's involuntary bouts of insane laughter. Igor slaps his cheek to do this, while the fastidious Mr. Bent pounds Hubert on the back.
Happens at least once in Dragonquest, when F'lar thinks he's about to die.
In Andre Norton's Forerunner Foray, Ziantha loses control when in Turan's tomb and doing a read. Iuban subjects her to this, quite viciously.
In one of the later Garrett, P.I. novels, Belinda starts to lose it when she witnesses a disturbing supernatural vision (implied to be of her murdered mother). Garrett snaps her out of it by wrapping his coat around her head, cutting off her view of the alarming sight and startling her back to her senses. (Garrett himself admits he has no idea how he came up with the idea, but he'd once seen a man bring his dog out of a seizure that way.)
Harry gets quite upset at the concept of being possessed by Voldemort during his PTSD phase in book 5, at least until Ginny reminds him quite sharply that she's been there and done that, and it's not at all like what's happening to him.
In the last book, Remus Lupin offered his service toward Harry Potter while at the same time, leaving his wife and unborn son, believing that they will be better off without him. In response, Harry (who is an orphan, and had gone through miserable treatment from Dursleys), calmly and viciously points out his cowardice and preference to "die in glory" than become a good father. He's snapped, but it had the desirable effect: Remus returned to his wife afterwards.
In the Old Kingdom trilogy, Lirael has a dogbite-shaped scar on her leg from when her wangst was getting way too out-of-control and the Disreputable Dog decided to intervene. After that point she learned her lesson and decided to just deal with the situation she was in without too much whining.
In The Outside, a woman suddenly spaces out during the Pride of Jai's evacuation. Yasira has to slap her to get her moving again.
In Phoenix Rising, the heroine breaks down into hysterical laughter after her Rage Against the Heavens moment provokes an actual answer from her god, and her aunt brings her out of it with a slap.
The Planeteers: In "The Brain Stealers of Mars", Rod Blake starts laughing hysterically after encountering an alien disguised as Ted Penton. The real Ted Penton cracks him across the face to snap him out of it.
Prudence Penderhaus: After Prudence knocks Mrs. Shooster out while helping Cassius escape from the basement in 17 Marigold Lane, Cassius is crying, and Prudence can't get him to move. Bonnie slaps him in the face and says, "Cut it out, Shooster." Cassius calmly replies, "It's not funny." Quoting Fright Night (1985) calms him down enough to start walking.
The Speed of Sound has a self-inflicted version. Eddie slaps himself in the face to prevent himself from having upsetting thoughts.
In the Sweet Valley High book Nowhere to Run, a toddler is choking. Her older sister cannot wrench her away from her hysterical stepmother and remembers reading that when someone is hysterical, the best thing to do is slap them... and she does. The woman's stunned reaction gives the girl enough time to get the baby away from her and perform the Heimlich, saving the child's life.
In They Hosed Them Out by John 'Bede' Cusack, a Dr. Jerk declares that an airman who's just had a very public breakdown from combat stress is Lacking Moral Fibre and delivers a couple of ringing slaps to no effect. He desists on seeing the Death Glare from the airman's compatriots. Later Bede encounters a more professional doctor who arranges for Bede to be 'held back for observation' so he can recover from a particularly bad mission (his pilot had pressed on with a faulty engine instead of returning to base for fear of being labelled LMF, only to be killed when a flak ship caught them flying too low). The doctor feels he's not doing enough for the war effort and considers resigning to become a pilot himself — Bede tells him he's doing just fine where he is.
In the Warrior Cats: Dawn of the Clans book Path of Stars, Clear Sky starts to panic and grieve when he thinks that his friends have all betrayed him and left his mate to die. It takes Wind Runner raking her claws across his ears and telling him to sit up and stop acting like a kit to make him realize that they actually have a plan to save her.
In "Who Goes There?", McReady slaps Kinner's face "with a methodical one-two, one-two action" when he bursts into hysterical laughter upon finding out the Thing is (apparently) dead.
In Wizard and Glass, Cuthbert punches Roland to get him to focus on their mission instead of sneaking off with his girlfriend.
In The Wise Man's Fear, Bast panics when he realizes Kvothe spoke to the Cthaeh. The Chronicler snaps him out of it with a slap, which is the opposite of their usual interactions.
, but here's a sample:"Then a great and powerful wind tore the mountains apart and shattered the rocks before the LORD, but the LORD was not in the wind. After the wind there was an earthquake, but the LORD was not in the earthquake. After the earthquake came a fire, but the LORD was not in the fire. And after the fire came a gentle whisper."