I agreed to giving her a key because sometimes the nanny has things to do, and if we're at work, somebody has to be there. She stepped up so we wouldn't have to hire a second person. That was fine. We didn't mind the help. Somehow and some way the visits turned into overnight and extended stays.
Examples of what I can live with. If I'm working overnight, which isn't unusual, it's cool if she's there with my wife. I know they have their Lifetime movie nights. Saves me from crazy movie central. No issue. She comes over on Sunday's to watch that show Downton Abbey, maybe? Cool, but here's the problem. She doesn't leave that night or the following morning or even ask if we mind if she stays longer than expected. It's assumed that it's wanted by both of us. (If asked, the answer would be no from me every time.) She'll leave to go to work or visit her apartment to get clothes, and then she returns like it's her house. We get home around 6 or 7, and when we get there, she's already there. I didn't want to treat her like the hired help and tell her that her services were no longer needed for the evening. Maybe I should have, and this would be under control. As a goodwill gesture, we've asked if she wanted to stay for dinner. I thought it would be understood that after dinner, she could politely exit the stage on the left and enjoy her night-at her own apartment. Never panned out that way.
Seriously, you are right. Nothing gets the message across like police showing up. If she can't respect our house and rules, stay away. The code for the panel and the password with the company will be changed. If necessary, the locks, too.