Unfortunately, there isn't much recourse other than to wait until they get out of the property. If it appears they are going to be out soon, you might just need to be patient. If they are not planning to move out, then landlord-tenant laws apply and regular eviction process should be followed.

For future reference, you should always do a final walkthrough before closing to see that the house has been vacated and no additional damages have been incurred.

Once they are out, you would be entitled to fair market rent for the number of days they occupied the house past closing. You can pursue that in small claims court.