Protecting a clantag, if that's what you mean is rather simple:
You save the playername into a string and search that string for the clantag. If it found a result, you compare his username to a list (can be a TXT file). If the username is found in that list, the players is OK. If it isn't found, kick him.
Kinda same way as the first clanprotection part: if the players username is the same as the username in the list (or txt file), he can execute the commands.
If you already have a configurable ranking insim, this can be added easily. There are a lot ways to do this, but first I have to know of what kind of rankings you're talking about...
He means so only clan memebers can use the Tag in their nickname. A server password alone can't stop that. For an example, our CLC server only allows registered CLC team members to use a [CLC] tag in their nickname. If someone puts [CLC] as thier tag the insim app server boots them and prints the reason is "attempted hijackking", gets a laugh everytime.