They happen at weekends when they think there are no Admin bods about. Another site owned by a big magazine group used to have the same problem and was closed down as a result. It's off-putting for possible new posters, and irritating for regulars, but it proved impossible to prevent.
I assume only registered members can post. So for those that post spam, may I recommend to rebuke them, then subsequently revoke their registration/membership once they've got the message? That way we can prune their numbers.