Post Limit can be a great thing, especially to avoid people answering "lol", "rofl" or some silly smiley to each post they see. It's however not so easy to tune :
- if you limit per account, it's really easy to create a new one (just need of a new ip). The only solution to avoid it would be to authorize only registered members, which is for me not bearable.
- if you limit per ip, peoplecan always bypass this (and will do it) using anonymous proxies and other stuff like that (anonymisers). Moreover, multiple people sharing the same ip (using masquerading for example) have the risk to be locked without reason. Last but not least, most people use a dynamic ip, they just need to disconnect/reconnect to be able to spam again - it's really easy to automate this in a script.
As you can see, there is no easy solution... Strong moderation is a way, but quite tiring for the moderator (tried - not liked). Maybe the best way is still to use legal procedures for people suiting it (I mean living in a country where law permits it) : they broke the rules, posted racism and homophobians insults which are PUNISHED by the law (at least in France). But who the hell will have the might to attempt a trial to such a bunch of silly teens?