At least some of the spam we get is actually people registering. I don't think there's any way to get around that. I actually got one after he registered but before he posted anything. There have been cases of a banned spammer coming back with a slightly changed spelling of the user name. Then I ask admin to ban the IP address. Spammers tend to stay on their spam thread for a long time - hours in some cases. I suspect (but not sure) that people who reply to spam (or click on the embedded link) get their address captured by the spammer for further spam emails.

As I explained to someone else; When I get on in the morning, I hit the "New Post" button which brings up threads with any new postings. I've already deleted reported spam as I get emails of those. Now I scan the last one to post on each thread for potential spammers. If someone has replied to the spam post, I'll miss it. Since I don't (can't) read EVERY post, some stuff that should get deleted is missed all together.