Contractors are a dime a dozen and call them selves that if they only have a hammer and one nail. You need to be ware, check up on references, never pay up front unless for supplies, and watch every step of the way.
If you have paid this person, most likely you are out of luck. If you have not, then you could hold the remainder of the money as a carrot to him/her.
Be like a lawyer, know the answers befor you ask them and pepper the person with questions as you go as a test and for product and service control in quality.