my familymate have my gamecenter account, how to let him logout from it?

My cousin uses my gamecenter account on other device to play Clash of Clans, I use my iPhone 4s and my galaxy S4 to play that game.. and my cousin uses his iphone 5 to play on my account, hes annoying that he just enters the game and ruins my village and plays without my permission.

SO i want to get rid of this thing, i changed my Apple ID password and my email and i put the two-steps verfication.. nothing worked, he still can logs into my account