My personal favorites is along the lines of: "Are you hard of hearing, you stupid clown?" - no overly foul language and safe to use in most company.
Few will try selling stuff to someone hurling nothing but insults at them. There's really only one group of people this doesn't work on, and they're the bottom of the barrel trying to find victims on public transport and the like. If you figure out a way to reliably get rid of them, let me know!