Nobody read those shitposters in megathreads
So kill the root of the problem, instead of adding some other rule.
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.
you can lock/delete them, but that doesn't solve the problem. many more similar threads will pop up in their place the next day. and it's a lot of work for mods to stay on top of.
these bounty spamming farms are quite mechanical; i think some of them are even employing bots now. i think they'll immediately build new megathreads in short order.
Some people really hates spams usually to Megathreads, some people are just using it/posting tonit 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.
the only scalable (read: automated) solution i see here is self-locking threads after a certain number of pages. but it's probably a bad idea, as it will arbitrarily cut off ongoing discussion in the case of non-spam threads. otherwise moderators need to address it manually, which probably isn't sustainable for any real length of time.