Damage something he/she values financially, and make it known that you will keep going, and won't stop until you have cost them 10 times what they owe you.
this works sometimes, unless they are poor and don't give a fuck about their stuff
breaking all their windows out of their house in the wintertime is always hilarious
if they have a pet, threaten it. If they don't pay, throw a chunk of hamburger with some crushed up lightbulbs in it, into their yard. (I am always paranoid about this, I love my dog)