knock (one's) block off
To strike someone with great force, usually in the head. If you insult me like that again, I'll knock your block off!
knock (one's) head off
To strike one with great force, usually in the head. Usually used as a threat or a hypothetical action. If you insult me like that again, I'll knock your head off! That football nearly knocked my head off. I'm glad I ducked in time!