I have just had to block a guest from my clan because he was trolling, but as a result he has now started to PM all the members of my clan!
Is there anything I can do to stop him being a menace?
Encourage your clan to add the player to their ignore lists if they feel as though they're being harassed by said person. They'll give up eventually they always do
This type of issue usually dies out quickly, although we've had a few people that have done this type of thing for months (I sincerely hope this isn't going to be your experience!).
I would recommend informing your leaders and members of the issue so they are aware that someone is trying to cause problems and the reason for his initial ban (trolling). Best of luck!