That won't be an easy issue to handled with but I thought a credit system should at least help to prevent from spamming.
I thought we should first think of this question: 'why did users keep spamming?'.
* maybe they're not satisfied with your web service, or argue with you or someone
* maybe just want to increase the number of post to be on the top of the billboard
So if there's a credit system, the administrator can then define their own rule on postings. They have to manage the user levels & authorities, limitations to their communirty. For instance, you can define a new registered user a READ-ONLY authority during the first 3 days after they get registered. Or you can define not to accumulate the number of post if a message is less then 3 lines. Or to put bounds on the number of concurrent threads from the same IP, etc.
