Why does occupancy even matter in terms of the amount of rent? You're leasing the space. Maybe the argument is "wear and tear," or if utilities are included? Even so, $600/mo is a ridiculous amount even for an additional permanent adult tenant.
Something tells me that there might be more to this story. It sounds like the landlord wants these tenants out of the residence, and doesn't want to (or can't) go through an eviction proceeding. This nonsense amounts to "constructive eviction." Why does the landlord want these tenants out?