SpongeBob for me jumped the shark when he started being crammed down everybody's throats, and he became like a symbol of the Third Reich or something, like, "If you do not watch SpongeBob and enjoy it, you're a turncoat!" It was a stupid show to begin with, but when it suddenly became hype-hype-hype, that killed it even more, and actually made it impossible to even try to like it when you're basically forced to.
And if we're going to talk about programming blocks, I'd say when Cartoon Cartoon Fridays changed to Cartoon's Network's Fridays, and they scattered all these kids everywhere and had them mingle with some really ugly and poorly manipulated puppets (like that truck driving dog/bear thing, or that alley cat and lop-eyed cockroach), and they didn't even really play any good Cartoon Cartoons anymore either.