I tried blocking kbin.social because spammers are posting a lot of junk there but I couldn't find it while searching in "Add instance" search popup.
is it because it's not a lemmy instance?
It would be nice if we could.
It's an ActivityPub protocol implementing fediverse platform. It has the equivalent of lemmy's communities which it calls magazines, but it also has microblogging similar to mastadon mixed in.
As a result they sort of work together which is awesome, but also you get stuff like what OP is asking about.