if its alot of money take him to court or send some lads around, u can go to ur local county court or do it on line and apply in the small claims section for a court date, cost depends on how much is owed
when i use to work for a plumber he use to go around xmas eve, turn off there water and fill it with quick drying concrete, and by the time they knew it had set and would end up costing the custy a small fortune to get fixed,
i know a bloke who got screw out of a couple of grand, so left it for a few years, then wrote you bas$ard on his front wall and put a card throught saying i thought you loved me and i find out ur married, cost teh bloke his marriage, all over a couple of grand
i am not suggesting that you do stuff like this, but if you got no chance of getting your money back, it will bring a smile to your face when you think back to it