If your lease limits the number of guest nights, you could look to enforce the lease against your roommate. You could potentially sue him yourself, since you are both parties to the lease, to enforce the term; or you could try complaining to your landlord, documenting the lease violation for the landlord, in which case, the landlord may serve him with a notice to cease, or stop doing this; if he does not, the landlord may evict him. These are both drastic steps with considerable downside to you: in the first instance, you have the cost and acrimony of a lawsuit; in the second, you run the risk of having your roommate evicted, which leaves you effectively responsible for the full rent.,