Therefore I have a great deal of experience dealing with appalling grifters, insane salesmen, and horrible horrible missionaries.
I have two tactics for upsetting people enough to make them go away:
So learn some of the following phrases, altho you needn't remember their meanings or even pronounce them very faithfully. Just have something to patter. Then stop and point to yourself and say "Oy naht Unglish - Estooniahn!", then patter some more. If they try miming or speaking slowly, just look blankly at them, mutter, shake your head, and repeat "Oy naht Unglish".
If you have RealPlayer (a/k/a "RealOne") installed, just download this 66KB zip file of a RealPlayer presentation.
Other folks, just play these small (under 12KB each) mp3s: