You can't hope they stay out. Admins and moderators have to take active steps to push them out. I want to give some kudos to lemmy.world for getting out in front of this, and I want to name and shame sh.itjust.works for having a vote in their agora community almost a week ago and then proceeding to do nothing
It's why my primary instance of beehaw elected to just defederate and sort things out later instead of desperately trying to navigate a flood of scumbags. The key will be implementing better tools and getting more people on board for community curation
I received a modest amount of downvotes on the big R for saying something similar.
I also commented (well, asked a rhetorical question but definitely got answers) here a couple of days ago that places that claim to be for free thought and expression tend to be full of either harmful garbage or nonsense conspiracy theories.
All it's going to take is one instance that isn't defederated with a jailbait community or someone with a poorly trained militia planning an act of terrorism to get Lemmy or kbin painted with the "great place for pedos" or "Nazi haven" brush.
I think the thing that makes lemmy and kbin difficult concepts for normies will also protect it from blanket association with nazis if one instance is infiltrated. Most people don't fully understand the distinction between the shared protocol and a site's domain.
People might think "lemmy is full of nazis", but it won't mean anything to them if I say i'm hanging out on midwest.social. To them it'll just sound like a regional blog website.