I'm going to guess here. I don't like to guess, but it seems no one else is answering just yet anyway.
You have IP addresses blocked on your forums? Somehow, you are blocking this person. If they can access other N54 forums and not yours, the problem is at your forum. IP blocking does not work. It is best to remove all IP blocks as they tend to block valid users more than they do trolls. The trolls seems to know the tricks to get around IP blocking, valid users usually don't as they are good people and don't need to know how to get around an IP block.
The only real security for Network54 forums is to make your forum "members only". I know, it's a lot of work, but it is less work than dealing with trolls on a daily basis. The work is all up front. After that, it is fairly easy. There are several discussions on how to do this, and there is a website and/or post on this forum with all the instructions you need to make your forum more secure.
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