What to do if someone is logged into iCloud on your device and they won’t let you log out?

How would you handle this situation: a device belonging to you is taken by someone who knows your iCloud login credentials. The device is then logged out of your iCloud, and logged into theirs. You still have the device in your possession, but the other party refuses to relinquish the iCloud account off of the device (meaning you can’t download apps you’ve bought, update ANYTHING, or use find my iPhone properly, etc.). What can you do about that?