In the fifth episode of the first season, Shredder creates a retromutagen gun that can turn Splinter back to a human. Later on Shredder is about to use it on the Turtles, which will turn them back into ordinary turtles. Splinter emerges from the subway, and immediately throws his staff, destroying the gun.
Leonardo: Master, that device was your only hope of being human again!
Splinter: I had to destroy it to save you.
The Season 7 episode The Starchild had Splinter disguising himself as the titular childlike alien's father and convincing him to grow up as well as the now adult Quarx offering to use his powers to undo the damage he had caused to the other aliens on their homeworlds.