Use the report function if it’s clearly against the rules. Open a thread here if it’s worth discussing.

You can also dm one of directly if it’s more sensitive.

I’d probably message the moderators of this community or maybe the admin of your instance. You could also make a post about it here if you wanted to have a discussion about it and why it’s problematic. It might be beneficial in the long run so people don’t recreate similar communities without realising they’re not allowed.