Don't fake anything ie the hot chick idea
Take this seriously and they are forced to take you seriously. If you've ever goofed around and played along, so to speak, you lose credibility. They can just say they were joking and you were part of it. Sure, the line can be crossed eventually and there may be consequences (ahem Incognito) but if you've always shown that you don't approve of the behaviour, its bullying and cannot be viewed in any other manner.