If they’re doing that, report them. If you see a listing in the dungeon/raid finder that specifically says “X realm not wanted” or anything remotely like that, report them.
It is actually against the rules to “voice” their discrimination against regions/realms/players like that. People can decline group members or kick them for any kind of reason, but if they’re being foolish enough to admit in chat or a group listing that “X realm not wanted”, that absolutely is reportable and people have been sanctioned for in the past.
This is a bit dated, but unless our local resident centaur pops in to correct me - it’s still relevant in today’s game.
And…normally I screenshot stuff to save someone from having to go look themselves, but I appear to have lost my trust level at the moment. So here is the link to the post itself. There are several posts from one of our SFAs, Vrak, about how it is against the rules to openly state the discrimination against other players in the discussion with someone who was sanctioned for it.