s, that's who/whom.

Sorry, there should be "Noun" in angle brackets at the beginning...

You have given us only a fragment, so it's hard to say for certain. However, it's unlikely to be "that's whom".

Sorry, full sentence was: "Weirdos, that's who/whom"

fivejedjonYou have given us only a fragment, so it's hard to say for certain. However, it's unlikely to be "that's whom".

Try writing out the sentence in full.

You seem to be reluctant to give us sufficient context. To answer your question effectively, we need to know what has gone on before. However, in practice, 'who' is almost certainly acceptable, 'Whom' may be incorrect; even if it is grammatically correct in some contexts, it sounds unnatural to many.

Ok, it was: "What man, woman or child doesn't like zombies? Weirdos, that's who/whom."