A few ideas that I would suggest in preventing a similar incident in the future:
1. (if you don't have this already) Turn on the broadcast to show when a guest enters the CC
- If necessary, you can make a KOS list somewhere (not on public forums) for players who are not banned, but on personal ignore lists, or for other various reasons
2. Give more ranks the ability to kick guests from the CC
- This permission can be given to all ranks (as far as I'm aware), so extending this permission to more of your members could result in problematical guests being dealt with more swiftly
3. Keep your ignore list limited
- If you "really" need to have someone on your ignore list, by all means leave them there. But, if you are able to withstand them without needing the ignore list, I would recommend having as few people ignored as possible in order to see situations like this in full. This is completely my opinion though as I do not have many ignores for this reason.
4. Set your online status to friends if this person is an issue to you in private messages
- This would allow you to be relieved of harassing PMs and free up that space on your ignore list. I find ignoring most people redundant as they can still find what world I'm on, even if I have them ignored and my online status is on.
5. Lock your CC to clan members only
- I wouldn't recommend doing this unless the only ones who guest in your CC are those who wish to cause disruptions, but it's still an option if necessary.
I hope that your clan is able to combat these issues should they be a recurring problem.
My clan had a "no guests" policy for nearly 2 years before we finally agreed to reopen it, though we do enforce strict rules on guests in order to prevent situations like these. And I strongly feel that we made the right decision in reopening it last year. We get the occasional troll here and there, but overall it's much better.