We hired a contractor to do work on our home. He provided an estimate. However did additional work on home with out our approval and caused damage to 3 places on our roof. Still has not completed the work but sent us an invoice. When we refused to pay for the work since it is not completed and damages he claims he can start billing up for past due invoice. He also returned to our home 2 weeks after claiming to have completed the work and did additional damage to our roof and tried to fix one of the items. He is now threatening up with a mechanics lien. Suggestions on how to proceed.