Infinity Ward’s Robert Bowling has announced that Clan Tag validation is going into the game in the next update. That means you will no longer have the ability to put illegal symbols into your tag. If you have a clan tag with such symbols, I recommend removing them or quitting the clan altogether as any form of bug abuse is taboo in online gaming.
I wonder why it’s even disallowed really, it’s not that big a deal but if something is not allowed to be done, it’s often not allowed for a good reason. Getting hit by a banhammer because you think you’re entitled to something via a bug is not worth it. Especially in a competitive arena where everyone is looking for a way to ruin you.