Okay, brainstorming how you might handle that sort of situation! (comments, made after brainstorming, in parentheses)

Leave the room. (no fun at all, but a good last resort, especially if you can bring boyfriend with)
Start a conversation with a different group of people.
Directly and politely state that you think he should stick to claims we don't have overwhelming evidence against, and if told that criticism is not allowed, say "okay, sorry," and leave it at that. (this is probably what I'd do first)
Start a conversation about tests of the supernatural and the boundaries of knowledge, framed positively. (if there was enough interest)
Go "GRaAAh" every time the supernatural is brought up. Perhaps use a spray bottle for negative reinforcement. (if only)
Be super sarcastic. (yeah, that'll work)
Be very honest. (a reasonable option - could just say that you feel frustrated when supernaturalism is brought up, because you don't want to seem critical but he just won't stop talking about it)