We don't want to stifle new users from posting, that's why we don't approve each person individually.
For the spammers we normally review the queue and their posts are automatically flagged pending approval when they meet certain conditions, or post the same message too many times, but this allows spammers that post only one or two messages to get through. We still delete those but the messages stay up for a few hours.
Right now there's between 1000-2000 spam messages that get deleted weekly before they reach the forum. What gets through is a very small fraction.