Can someone try to bind you to an illegal contract?

A roofer we decided not to use, because he was featured on the news for fraud, and issuing illegal contracts, has tried to bill us for 5,600.00 to a broken contract. The contract we signed was not filled in all the way and no price for services were stipulated, so how can he try to demand us to pay something we dont owe. We decided to use a roofing company that was legitimate and not featured on the news for fraud. We cancelled the contract on 10-31-09 and he sent the letter on 11-06-09. The Dallas County Sheriff's Dept. is looking for him, that's what it shows on KHOU news video.