I can't remember the sign up for the site, it's been a long time from when i first signed up, and you are right the last 2 points I made would not make the board user friendly.
I post count limit on new members may be an idea, lets say 10 posts before you can post a link on the board.
But if you do that then there is the chance they will make 10 posts saying somthing silly like "interesting" in every posts, thus adding to the spam.
Or you could mod every post from new member lets say 5 post, this means that every new member that signs up and makes a post to the forum, they have to have it checked out by the mods and admins on the site.
if you use the Mod the first 5 posts, and also restrict the PM so that a user has to make 10 posts, this will slow the spam down a lot.
Not only will they have there first 5 posts checked before there got on the forum, they will then have to make 5 more posts before they can send a PM, this means that they can't send PMs if they can't post to the forum.
you can find a lot of Mods to help with spam on this site:
http://www.vbulletin.org/forum/forumdisplay.php?f=198
So have a look and see wat you can come up with