When I really want to insult someone, I just tell them to crawl back into the sewer they came out of. That and 'scum' or 'scumbag', seems to get the insult across hard and swift, like a one-two punch to the face or a kick to the nuts. They rarely come back for more once you tell them how fugly they are or how much they smell like ass from living in a sewer.

Works like a charm.