Blocking new users for a few days can be counter-productive and frustrating for genuine new users.
On other forums where I am a moderator or have some admin rights, various other methods are used, such as severely limiting the number of new posts (but not responses) per day for the first few days/weeks of membership, or all new posts for the first few days of membership need to be moderated and approved before they appear in the forum. The SPAM attack at the end of last week seemed to involve only a couple of dozen posts, but I do not know if it was auto-limited and if the attack was intended to include a vast number more, or not