Subtle shenanigans could go for some time without being noticed.

And wouldn't people who can code themselves host their own javascript, rather than hotlinking someone else's?

This is what I was thinking, going all stuxnet on them and just subtly moving the advantage your way. The outlier is the propeller guys, they look like a site that hired a 3rd party team to build them a website, and that team ripped of this one and made it theirs. Screwing with it isn't going to send the right signal.

"Assuming that the people who are copying this code are able to code themselves"

Probably a generous assumption.

If you broke it in such a way that it made their site massively unprofessional or completely unusable, then they'd be forced to either redesign their site or take it offline. Either of which results in those sites no longer feeding off copied code.