It could probably be done by blocking his/her IP adress, but he could probably get by it by using another computer or going to a website like "anynomizer.com". This is all assuming he doesn't use a dial-up connection in which case the only was to block him would be to not allow posts with "Nostrildomus" in the name line. In that case, he could just change the name he uses on A4.org . . .

KLHI
klhi@ibm.net
99.0 2.8QM Melange/Onyx Cloth