If you put in an offensive tag or emblem and multiple people report it, that part of your combat record will be temporarily disabled. There is a thread about this in the Treyarch forums but they do mention that it is only for 3 hrs

ok so i just prestiged and got to lvl 6 so i can use a clan tag, i type in Aus and it says"clan name unvailable due to prohibited text i tried every letter invidiually 2 "a" "b" and still not working. anyone else having trouble with this?

Dude, just contact Treyarch. I know it's silly contacting them due to an issue with a clan tag, but It'll help them solve the actual bug (I'd bet $100 your not the only one that can't put a clan tag).

if you change you xbox live name it will work i had the same problem and i changed my name and now i can make one

www.youtube.com/watch?feature=player_embedded&v=a1Y73sPHKxw

Haha, "Aus" clan name unvailable due to prohibited text.. lol'd