I was told by a lot of people (including my attorney) that I might have to let my ex stay until the divorce was final (in my state the waiting period if you have children and can prove that the other party was served is 6 months). I decided to hell with that, he was going. Sometimes you have to get creative. I was told I couldn't change the locks after he did leave and I changed them anyway. I told my ex that if he did not sign the agreement I would ask for a friend of the court investigation and his "condition" would be on record. Does your STBX have a job? Does he pay CS? In my state not paying CS means JAIL.
I think you may need a 2nd legal opinion.