He kept posting his long angry rants for a year before he realised what was going on. To say he was upset would be an understatement. I'm sure he tried to track down the site owners and admins, but we were very careful to maintain anonymity. So he went after regular members, he managed to figure out where some of them lived and worked by analysing their posting. One day he posted the names and address of a lot of the forums top posters.
Of course at that point the police were called in. The guy was tracked down via his IP address, he made no attempt to hide his activities. We did send occasional abuse reports to his ISP before this happened and they were always ignored.
He was charged with harassment and ended up with 100 hours of community service and a £150 fine. A pitifully small punishment given the fear he caused and that he did not show any remorse. If anything he seemed annoyed that he got caught. Apparently, in his first Police interview he argued that he was merely exercising his right to freedom of speech and was surprised when the police told him that such a right did not exist in the UK.
Some people have anger issues, some people have real mean streaks. Forums attract these kind and can encourage them to act their worst. If someone goes too far and starts to make people feel personally threatened or tries to ruin a business then I don't see the problem with them facing a severe penalty when caught.