Store them in an XML file(?)
So what, you have a list of valid names that anyone can use and get Clantag access? I think not
I'm a n00b here, so excuse me if I've got this wrong - I've not actually started scripting yet but given what I'm reading in the wiki, I would assume that you would make use of Account functions on the server to securely tell whether a user belongs to a clan?
Obviously you would need to verify that they belong to the clan first, e.g. the user would register on the server using addAccount etc, and then they apply to become part of a clan. It would make sense that you then need an Admin or "1st officer" of the clan to accept them into it, as you would expect in any clan/gang. This clan membership could then be stored against the User's Account, using setAccountData(userAccount,"clanMembership") or whatever you want your key to be called.
When the user logs in, instead of using their nick to verify them, it would actually require the user to login (with username/password), at which point you can then check getAccountData(userAccounts, "clanMembership") to return whether the user belongs to the clan you want them to be part of. Once you know if they belong to the clan or not, kick them if their nickname gives the impression that they are, by using the pattern matching that was mentioned earlier in the thread.