I have a contactor who sent me several change orders for work that I never agreed to pay for after the work was already completed. There is no signed contract or estimate for the work. He is now threatening to file a lien on my house if I don't pay the extra bills that I never agreed to or knew were coming until after the work was complete. Can he actually file a lien on my house for this, and what rights or recourse do I have to stop this?