I like the idea of locking the dog in your roommate's room for the day, and then let him deal with the mess. My suggestion would be to arrange for a friend (who can keep his mouth shut) to keep the dog for a couple of days. When your roommate comes home, tell him that since he wasn't able to care for the animal, you had Animal Control take it to the pound. Let him sweat it out for a couple of days trying to find the dog, then bring the dog back. Tell him that if he doesn't straighten up his act, the next time it will be for real.