Blocking IP addresses does not work ,as Ripper points out.
To block your trouble maker, you basically have two choices,as best as I can determine.
1. Do as Ripper suggested and set your forum to "Members Only."
2. Create two groups and set the access codes for those groups.Your trouble maker would be in one of those groups,with his/her access code set to Read Only.Put everybody else in the other group and set the access code to Post HTML and/or Revise.Set the public to Read Only and the Network54 members to Post Moderated. When a new member shows up, put each one in your big group.
If the above solutions don't work for you, you might try this:http://www.network54.com/Forum/44985/message/1071333819/When+it+happens+to+me+%28rare%29
Pick a few of your trusted/daily members,put them in your moderator group(or create one)and give them Edit/Delete access power. When they see one of your trouble maker's post,have them immediately delete it without making a comment. Hopefully, after a while he/she will get tired of being ignored/deleted and leave for good.
Please let me know if you need help;feel free to contact me