Unusually, I'm not with you on this one.
"I'm sorry, that particular build is too powerful. Please choose something else," is an entirely reasonable thing for a GM to say. So, I'm fine with having the a list of known offender combinations, or case-by-case decisions on particular characters.
I would not be so keen on, "I will only allow one multiclass character at all, you may choose who gets it," mostly because I don't see how this kind of restriction really prevents whatever problems might arise.