Un-assigned person sleeping in house regularly

I have a house, let on individual room basis. Due to one of the bedroom doors being kicked off its hinges and room trashed, I decided not to replace the door immediately to allow consideration of what refurbishment work I may carry out.

I entered the property with a builder to find a 'friend' of an existing tenant had clearly been sleeping and using this trashed room, although someone had taken it upon themselves to clean all the broken glass up!

I gave him a warning not to return and under no circumstances was he allowed to use this space. He denied he was sleeping there, but having felt the mattress, found the bed was still warm, bedroom curtains closed and personal belongings evident. I had spotted him leaving the property on several occasions very early on a morning, when confronted previously he said he had just called by to see his friend.

My builder said I would need to tread carefully. Is this right? Can't I stop someone living rent free and using empty rooms? All other empty rooms are locked but as this doesnt have a door, there is nothing to lock!

I have warned the existing tenant not to allow his 'friend' to sleep or use this space. Am I right to do this also?