Choke: When Frankenstein's Monster hits a single opposing character with a close combat attack, that character can't be moved or placed until your next turn.

What happens if Frankenstein's Monster rolls knock back? Will Choke prevent the character from being knocked back?
Yes. Knock back does not activate effects that normally activate when a character moves, but there is nothing to activate here. Instead, it just says the character can't move. So it doesn't.

Does that mean the character would take knock back damage, then?
No. Knock back damage occurs when movement is blocked. This effect does not block movement; it prevents movement from ever happening at all.