It's a difficult issue and sometimes the lines get blurred, I'll give Cameron Bruce and Adem Yze as an example. We've all seen the miltitude of negative threads and posts towards these guys, mostly they're content like 'can't kick' or other such critiques, but often they transcend down to another level with accusations of softness and alike. This type of insult could definitely be taken as a slur against their character. Where to draw the line is the question, because one day an internet forum will be the cause of some sort of legal action taking place.
Generally the 'mood' of a forum is generated by the mods. If mods are abrasive and aggresive then the posters will respond in kind - I'll give bigfooty as an example. There was a mod there by the name of Porthous who definitely fitted into that category. He was a mod in the Port Adelaide forum but his signature carried the mod title.
Often he'd be seen having one on one very personal and insulting arguments with others on the main board, now other posters would witness this and think that because a mod is undertaking in this behavoir then that is the acceptable level to post at, which really isn't unreasonable. This standard of posting eventually gets directed at the players.
If the mods behave responsibly in the first place then policing attitudes is a much easier task, and I know from personan experience because I've been one before.