It can be incredibly frustrating dealing with bad roommates. If you are renting all or a portion of your apartment to someone not included in your original lease, you are deemed a sublessor or landlord to the roommate/new tenant. The sublessee then becomes your tenant, with all the rights and protections that you enjoy as a tenant with your landlord.
Whomever is on the original lease with the landlord remains fully responsible for the rent to the landlord, regardless of the payment or nonpayment by a co-tenant, roommate, or sublessee.
The only way that you can lawfully remove a sublessee is through an eviction or summary process action.
By subletting and becoming a landlord, it's very important to determine whether you complied with all the landlord-tenant laws, especially if your took a security deposit or last month's rent. If you didn't do so, the sublessee may have several causes of action against you that can become very expensive, very quickly.
If you would like legal help with an eviction, feel free to contact me.