Tenant's Wall Mount for TV

[Lacey A.]

Our signed lease clearly states no use of nails or screws of any kind in the walls. (We did this because the drywall is super old and paper-thin.) In our 6 month inspection, we saw that the tenant has blatantly used nails to hang artwork/pictures throughout the house and they installed a wall mount for a TV.  Upon move out they are responsible to make any necessary repairs. We are inclined to have them leave the wall mount in exchange for no repair fee because the holes will be very difficult to repair to look like it originally did. 

Two questions;

Do we just let them try to repair and fine them if it's unsuitable? 

OR If we were to keep the mount for future tenants? But are we liable if their TV fell and broke, or worse, hurt someone?