Unfortunately, as far as I know, your only option is to set the contract to not renew and wait for it to expire. Once it expires, the vendor will vacate, and the person will have a limited amount of time to claim gold & goods from the house sign. When the claim period is over, gold & goods will go to the house moving crate and you will be able to access it, so you can return it if the person ever shows up again should you be so inclined.
Once the vendor is gone, you can customize the house.
NOTE: You can customize when the vendors are your own, it is just rented ones to others that stop you from customizing.
-Skylark