The only solution that I can see to avoid these "shitposting" in Megathreads is, Delete all Megathreads. As we can see, these Megathreads are already worthless, they're just like a trashcan full of garbage and the only solution to clean up all the mess is to dispose it.

Some people really hates spams usually to Megathreads, some people are just using it/posting on it in order to increase their activity and not worrying about the quality of the posts. But why don't we kill the root? Which is these existing spam flooded threads.