In most instances, an unauthorized tenant can be evicted from an apartment with or without cause according to Nolo.com — a legal information clearinghouse. In some states, authorized tenants may need to proceed with a formal eviction by acting as a master tenant, although this is not always the case.
Even if there is no written agreement between the unwanted person and the person leasing the apartment or between the unwanted person and the landlord, the unauthorized tenant may have grounds to fight an eviction after living there for a certain period of time, so it may be necessary to consult the local housing authority or even a lawyer about potential remedies.
It may boil down to a need for the landlord to evict the person. The landlord may even need to file a trespassing complaint against the person, although the person may make the argument that the landlord consented to the tenancy through inaction (in other words, the landlord knew of the unauthorized person living in the apartment but failed to take action to remove the person).
In rent-controlled properties, or in areas where there must be a good reason for an eviction, it can be tricky to evict just one person, and as a result, some landlords may decide to just evict everyone in the unit. In the case of a roommate who poses harm to other tenants or occupants of a unit, a restraining order may be necessary and a judge may ask the person to leave the premises immediately based on information presented.