。
Quit cross-posting, idiot.
Literal
Multi-post do-[don't-な]. Idiot.
マルチ here is the net-slang abbreviation for マルチポスト 'to multi-post / cross-post,' the forum offense of dropping the same message into multiple boards or threads. The bare-imperative prohibitive ~な, suffixed to a dictionary-form verb, is the harshest way to say 'don't X' — blunt, contemptuous, and exactly matched here by the tacked-on アホ 'idiot.' A tiny complete utterance that's almost pure tone.