Aaronrulz, glad to hear this infuriating issue hasn't manifested for you yet. It happens with my one and two star celebs all the time. It also happens when my non-celebs go clubbing and Venessa or Izzy or whoever show up there. My workarounds are:
1) Turn on stereo instead so sims can carry on dancing (but music choice is limited)
2) Get any sim for whom it's vaguely in character to learn how to DJ, so they can take over (practically all my celebs have 6-plus skill)
3) Give up and take my sims off to the bar when Venessa or whoever shows up.
SimmerGeorge, thanks for the tip and I hope others see it. I play vanilla myself but it's good you're spreading the word about this helpful mod. I also can't fathom why the devs don't turn up professionalism a notch for DJs and mixologists, although I do find the latter more diligent and my sims do usually manage to get served. But do DJs actually behave like this irl when celebs show up at clubs? Wouldn't they get fired for not playing any music?! Wouldn't any club that stopped playing music for hours at a time so people couldn't dance go out of business??! I Just Don't Get It. I have considered posting on the feedback discussion about this, but like you say the devs are never going to fix this so is there any point.