In chapter 17 of Lightning and Thunder, Voldemort got a little testy with Harry's attempts at stalling during the Philosopher's Stone confrontation. "I. Need. The. Stone."
In chapter 5 of The Change, Draco Malfoy was exhibiting creepy stalkerish behavior and Hermione led him back to the Slytherin table, stating "Stay there. Leave. Harry. Alone." In a later chapter, Fudge came to the school to see Harry and when he refused to give a reason, McGonagall said "Stop. Right. There."
In chapter 3 of A New Addition, Harry fled to Grimmauld Place after discovering Ron and Hermione were homophobic and when Snape and Draco Malfoy found the latter two on the Quidditch pitch Snape grabbed Ron's robe and said "Have. You. Seen. Harry. Potter?"
In chapter 10 of Night Flight, after getting a reluctant Zabini house elf to agree to obey his orders Harry added "And afterwards I don't. Want. You. To punish. Yourself."
In chapter 16 of Passageways, McGonagall became fed up with Dumbledore's manipulations in Harry's life. "So again, I say to you Albus Dumbledore: LEAVE. HIM. BE!"
In chapter 4 of The Heir, Harry was annoyed by Ron Weasley's stalker tendencies and snapped "We. Are. Not. Friends."
In chapter 12 of His Angel, Cedric kept trying to push Harry into taking the Triwizard cup and Harry finally snapped "I. Don't. Want. It."
In chapter 10 of Taking Back What Is Mine Snape found out that Dumbledore didn't give Harry any basic Occlumency instruction before Snape's lessons and raged "I. Am. Going. To. Murder. Albus. Dumbledore!"
In chapter 21 of A Marauder's Plan Sirius snapped at a particularly stubborn Dumbledore "Harry. Will. Never. Go. Back. To. The. Dursleys."
In chapter 11 of Amber and Emerald Harry stabbed Tom Riddle's diary multiple times while muttering "Shut. The. Hell. Up. And. Die."
Red Lightning has Double-D deliver a beautiful speech that's constructed ENTIRELY! LIKE! THIS! When he gets to the clincher, he managed to get a freaking ARMY of deranged Super-Psychos to shut up and listen to him.
Bridgette: I am going to make you eat… every… word.
Roy Mustang delivers a subtle one of these in Brilliancy. He's in the early stages of a debilitating fever and it's just been made clear that his Living Emotional Crutch has been kidnapped, so the narrating character notes that it's pretty much costing him every ounce of strength just to talk.
Find. My. Wife.
In A Great Endeavor, "Purple. Pony. Girl!" is General McAuliffe's disbelieving response to seeing Twilight.