He's having a laugh to think he can get away with this once, let alone monthly! Surely he'd be violating your lease agreement in some way? It's worth looking over it to try and find any specific wording that supports your position. If you haven't already, itemise the problems that you & your other roommates have with what he's doing, and then go over them together with him. You need to establish ground rules and make it clear that if he doesn't agree with them, it's not the right house for him. Make sure you keep a record including dates (not just from now on, but going back) of all issues/attempts to resolve conflict/any other relevant communication, etc. At the end of the day, the leasing agent/owner is more inclined to be on your side (not wanting to expose the property to risk), but I'd also keep an eye out for any potential new roommates so that you can offer an alternative, paying tenant ready to go if necessary. Is his music any good, at least?