Technically, only a landlord can evict a tenant who has signed a lease. However, there are some circumstances where an occupant of an apartment can be considered an equal tenant with those who have signed the lease.
A person is considered a tenant of an apartment if she signs the lease. A person may also be considered a legal tenant if she has lived in the apartment more than 30 days, paid rent directly to the landlord or had her name put on the mailbox.
A landlord is the only one who can evict a tenant. If your roommate is a legal tenant you cannot evict her, even if her name is not on the lease. However, the landlord may evict one tenant and let other tenants remain.
A landlord must have compelling and documented reasons to evict a tenant, such as failure to pay rent.
If your roommate is not considered a tenant but refuses to leave, perhaps your landlord can act as a mediator to help settle the dispute.
You may have to hire a lawyer to act on your behalf to help remove your roommate if he is not a legal tenant.