Flood controls on new members can help*, but it usually means the spammers change tactics and it slows down the clean-up. It's quicker to remove one spam account and the 50 posts they've hit the forum in one go than it is to delete 50 spam accounts that have made one post each. Likewise for members to tidy up their view of the forum by only having to Ignore one account rather than 50.
Unfortunately the truly effective controls remain manual and tiresome. Having Admin in multiple time zones generally helps with response times, but unless you're looking for something to moan about there hasn't been any major inconvenience caused by these attacks to date. It's been obvious that they're spam, it's easy to ignore them.
* more effective is preventing probationary accounts from including hyperlinks until certain milestone achievements have been made by the account (not just days served or posts made, the forum already has an appropriate mechanism in the points system). It protects anyone foolish enough to follow a spam link, removes some of the attractiveness of the forum as a target, and requires a specific and unique bot script just for this one forum.