The first thing I have to say about dealing with spam links is that most spam that is posted on sites are posted by robots, and they will not read your rules! Don't expect bots to follow rules. A lot of people seem to believe there are real people behind these types of posts, but it's just a computer, and it is not going to abide by rules because it's not programmed for that. Don't get offended or post a bunch of announcements about it, because it won't read them and doesn't know how. There are other measures you can take.

Basic Guidelines for Dealing with Spambots:
1) Email validation does help some against spambots, but admin validation generally turns away more members than it helps your site. The disadvantages are greater for admin validation than the benefits, so it might be worth dealing with a few bots if you get more members from simple email validation.

2) Check out the support forum for your forum software. To find this, generally there's a link to the main website at the bottom of your forum (for example, InvisionFree's is at the bottom of this forum). Click on the link, and then on "Support". It might help then to search for "bots" or "spambots" on that forum.

3) Have some active moderators that come on at different times of the day. Moderators and admins are the best measure against bot posts. Just delete the posts (or archive them). Have a thread in the staff forum which records the name and email, and then add that to your ban list.

4) Look for a ban list on your support forum's website. Input this list into your banned IP addresses or email list, and this will help a lot.