How to prevent those people from joining? I don’t think you can.
On the other hand, Reddit communities never got that terrible, right? Not all of them at least - it’s more that the platform turned to shit. Lemmy prevents that from happening. The concept of communities moderating themselves seems to work pretty well.
Unrelated question, how does Piefed differ from Lemmy? Is it designed to exist alongside Lemmy, or is it a better alternative somehow?