How can you prevent someone from seeing your messages if they sign into your Apple id

You can't. You need to change your password so they can't access your account.

Apple ID Password Change

What to do after you change your Apple ID or password - Apple Support I suggest doing this before you change your Apple ID or password.

Well how can you siign some other than yourself out of your Id . They have it tied up for months now yet support just keeps saying there’s 3 ids that are there and they are very similar and until they have figure it out I’m in recovery for 9 more days, yet other things as well going on with this iPhone. Time to get others phones and other things .

Are you able to sign into your Apple ID? Can you change your password?