I definitely like this idea, as trolls will be trolls. However, I don't want to limit who can put on any given clan tag.
People should still be able to put on random clan tag names, as long as it's not already "used." If Jimmy and his 3 friends who all bought the game together want to put on a random clan tag name, I say let them. I think there should be an official "dibs" system, where you can officially claim the clan tag "Gent," "WOLF," "COx," ect. In order to prevent people from spamming this "dibs" system and requesting every possible combination of 4 letters there is, why not monetize it? I feel like most competitive clans wouldn't mind dropping lets say $3 to gain proprietary rights to their clan tag, but this would be enough to stop random people from requesting official rights for their tag. Whoever bought the clan tag could then give out permission to use the tag to his/her clan members, but no one else could use it. Jimmy and his friends could still use the tag "BOOM" as long as it didn't conflict with a clan that already bought out the rights to the name. He wouldn't be made to pay anything for it, unless he wanted to make a competitive "BOOM" clan and recruit others but prevent random players from having the tag. Regular tags would be (in parentheses), bought out tags would be [in brackets].
I don't know, it's late and I've had a few tonight so this might be a terrible idea. Let me know if this is completely crazy.