As long as they keep their opinions in whatever community they choose and don't force it down anyone's throat, I'm fine with it existing. I'd rather have ugly opinions openly debated instead of festering in the background where others get sucked in and we get a pathway to extremism. It's pretty easy to just block the odd community that pops up all you never see it again.
Debate them or ignore them, but completely blocking them just kneecaps this burgeoning platform. Nobody is going to join if the standard is "only leftists that meet this litmus test".
Obviously you can vote however you choose, I personally prefer to lean on the side of allowing content vs blocking it outright, at least until they refuse to obey our rules when in our communities.