Someone is using my Game Center account; how can I secure it? I shared my Game Center details with my friend. He betrayed me and changed the primary email address of my Apple ID.

When I persuaded him a lot he gave back my Apple ID and Game Center and I quickly changed all my info. However I have found out that he still uses my Game Center.

I have another Game Center ID and I saw my own Game Center ID active 2 hours ago when I woke up in the morning. How can I log off the other device from my Game Center?
I guess his device is still logged in with your credentials, and even if  you changed the email address or password, his device is still mantaining the session active.

I don't think there's a way to log him off remotely. It seems to me that the only way is a manual log-out on his device.