Hi Sophie,
So I'm assuming you had a guest with no verifications ask to book, you pre-approved, then withdrew the pre-approval, but since you had instant booking on, the guest got verified and booked anyway?
My best bet would be to try communicate with the guest expectations and standards you require them to abide by throughout your stay. Get them to tell them a little bit abotu themselves and inform them of the house rules. If the reservation is really a concern to you, then maybe ask them if they would like to cancel. Cancelling a confirmed booking really hurts you in the long run so isn't really advisable. Best option is to try work with guest and inform them of what's required.
Good luck!