Did you ever sign a lease with them? Is there any record of payments anywhere? Then you could possibly go to the cops and say “I have these guests that refuse to leave my home. They have overstayed their welcome and refuse to leave. Can you help me out?”
If it is NOT a legal apartment it might very well be easier for you to get rid of them. Because you are allowed to use such space for yourself and your guests but you are NOT allowed to rent it out. So if you are tired of your guests and they refuse to leave, the cops might help you get rid of them.