My members have a cure for members like that. It's called the silent treatment.
If you can code, here's a couple of thoughts on how to start this one.
1. Add an invisible field to the user profile that you can use to set the limit.
2. In the newreply.php and newthread.php, check to see if they have a posting limit. If they do, do a query to see how many posts they have made in the last 24 hours.
3. If they have exceeded their quota, show them a template that says they can't post.
This way, you are only moderating trouble users and not everybody.
Good luck with it. It should be a relatively easy hack for you to do.
Amy