If the tenants are violating the terms of the lease, you could send them a notice informing them that they are violation of the lease and must cease the violation and giving them at least three days to cure the violation--i.e. at least three days to stop using the extra space. (I recommend a few extra days in the notice, to make sure that if they get the notice late, they still had at least three days.) The notice should also state that if they don't comply, their tenancy will be terminated and they will be evicted.  Send the notice several ways, including some ways you can prove delivery; e.g. by regular mail and also certified mail, return receipt requested; you can also hand deliver a copy. If they don't comply, then at the end of the notice period, you can file an eviction, or "unlawful detainer," lawsuit. This is the only way to enforce this sort of lease obligation: with the threat of eviction. If you are unwillng to evict, it's not worth going down this route.Landord-tenant law can be fairly technical: you are advised to preferentially retain a landlord-tenant attorney to help you.