If there was no agreement to hire them to do this--i.e. no agreement to pay them for replacing your screens--then legally, you do not owe them any money: no one can come to your place, do work without a contract or other agreement, then force you to pay.Practically, even though on the facts you describe, you'd likely win in court, they *could* file a lawsuit (it's almost impossible to stop someone from filing a lawsuit) for the amount they believe they are owed and force you to spend and possibly money (if you hired an attorney) defending the suit. Depending the quality of the work and what the screens cost, you may wish to try to settle with them rather than go through litigation.