How do I evict a roommate who is not on the lease?

If you have a roommate who didn't sign the lease your landlord, then you are their landlord. So you have to evict according to the laws covering your area.

In short, just the roommate by himself cannot be evicted. See here (https://www.askmehelpdesk.com/real-estate-law/state-landlord-tenant-laws-other-helpful-information-25845.html#post144888).