Over the past couple of years I have had similar problems on several occasions. I have been dealing with a very slick and smart hacker. Applcations on the Apple are very easy to manage. Delete it and it completely clears it out. Add it back with proper appleID and you are in business. I would recommend that you change your password and turn on added safety features like 2 factor and facial recognition to further protect your data. You can even use Finder if working on a MacBook Pro to see if this other user ID is linked to your system in any way. If you see files containing an unknown user ID contact Apple Support for help with clean up.