I find that a strong enough insanity plea will chase off or at least give pause to even the most determined of bullies. Mind you that likely wouldn't work so well against, say, someone who was hired to kill you no matter what, but OP was vague enough for some wiggle room.
If they just want to beat/rob you,
1. [empty fist] What's in my hand? Guess correctly and I'll give you my prized fountain pen!
2. [crazed look in eyes] Have you ever tasted human blood? Give me your blood, give it to me! [act like I'm going to eat them]
3. Your shoe is untied.
or
4. Right into my trap! Get em, boys! [when he turns around, sweep his legs out from under him]
Alternately, if they do want to kill you, you might try:
1. Don't you recognize me? It's me, your long lost brother[/sister]? We were separated at birth, I've spent so long looking for you!
2. Idiot, your safety is on!
3. I'll pay you double what they're offering [and of course escape and hide asap]
or
4. You don't really want to shoot me [wave hand in front of their face]