You can't really deny them access to the sim. Best thing there, if there's truly a personality issue, would be to cancel the session and address the issue with management. They can take it up with the POI or higher if they concur.

Just because the guy was wrong once before doesn't mean you can turn him away. If he's repeatedly wrong you could address that pattern with management and the POI.

If you think he's biased or out to get you, you could address that with the company but you'd probably need some sort of evidence.

If it's an actual flight and the guy is intimidating or creating a distracting atmosphere that would seem to give safety grounds for denying him, but hopefully you have a witness and you'd better call your MOD on the spot before you go.

Tough situation. The bad ones can usually be intimidated if you know how, at least in a real airplane. It's not good for their careers if they create issues involving canx or delayed flights.

I had one fed who came on all hard and steely-eyed. It had already been a long day and I wasn't looking forward to it. We're getting ready to go and my FO chat's her up and asks if she's also making the return leg on our out-and-back. She answers no, she has some investment property at the destination and she's staying to check on it...she wasn't even 121, just a piston GA inspector. I told her to go sit in the back. Point being talk to them, see what you can learn.