It's one of those rules that is loosely enforced until somebody asks that the offending players remove the fake clan tags. If it bothers* a member of the actual clan, then they get forced to stop. Usually banned until name change request goes through, just like anyone else with an offensive name. I can't navigate the main website while I'm at work, but if nobody else takes care of it by the time I come home I'll do a thing.
*bother [both -er] verb - to troll mildly, or in some cases, in an extremely British way.