How can I keep Games Centre on but disable the popups whenever I unlock an achievement?

I'm playing Brutal Legend, and I've already got the achiement for killing 300 enemies, but now whenever I kill anyone then the achiement pops up again. Same with any other task you do that unlocks an achievement. The result is a constant stream of popups at the top of the screen telling me I've unlocked something which I'm already aware I've got.
I tried disabling notifications for Games Centre but it didn't work. The only way to stop it is by signing out of Games Centre, but I don't want to do that because then I can't unlock other things.
Help?