If I understand what you are saying, it is not the spout which is leaking, but there is a leak behind the spout where the copper tube extends through the wall. If that is the case, how about unscrewing the spout and caulk liberally around the tubing to seal that opening, then replace the tub spout. The caulk will make the seal, and it will not be necessary for the spout to seal against the wall.