Unfortunately, it probably isn't possible to. Unless, of course, everyone here (and I do mean everyone) is perfectly alright with the Fediverse never gaining mainstream popularity, the plain and inconvenient truth is that it's only a matter of time until Lemmy and Kbin are infected with the same kind of shit. This phenomenon predates Reddit, it predates 4chan, it predates Digg. Ask early Usenet members 30 years ago just how far back this issue goes.
But what if, instead of trying to prevent it entirely, we simply tried to slow it down as much as possible? Now, you're working with reality, not against it.
One idea I've always been in favor of has been the concept of installing limits: limited posts, limited replies, limited votes, etc. I don't know if this is a thing that could be rolled out on an instance-per-instance basis or that, even if it could be, if it would be as effective as a platform-wide initiative, but the appeal of setting limits is to introduce scarcity and thus more weight to a user's actions.
If you only have X number of possible actions per day, such as X number of posts, how might that affect your behavior? Would you still shitpost as often in every pun thread, upvote every repost, argue with every single troll? Probably not.
There are obviously some downsides to this as it might have a not insignificant effect on promoting genuinely good content and or punishing (downvoting to oblivion) objectively bad or offensive content -- and again, at best, you'd really just be delaying the inevitable as long as possible -- but I think it's worth investigating nevertheless.
I believe that's called a running mate, and not too many people seem keen on a President Harris.