How to evict the tenant who is occupying one room in a flat with few others?

I think the best bet is to discuss the situation with your Landlord.--I am sure that he doesn't want everyone to end up leaving and having an un-rented apartment. He will know the best way to proceed and hopefully help you. This is one of the draw-backs you have to deal with when sharing a place--and you really need to figure out a way to protect yourself by having some sort of written agreement with anyone in the future.