[agitated]

First steps that help to combat this (does not stop it) is to have email verification enabled at registration.

In the ACP -> vBulletin Options -> vBulletin Options ->Human Verification Options -> Verify at Registration = Yes
Now go to -> vBulletin Options -> vBulletin Options ->User Registration Options -> Verify Email address in Registration = Yes

Now in the ACP -> vBulletin Options -> Human Verification Manager
Set your verification options for -> Image verification

Consider turning off the Memberlist for all until you get things settled.

You can strengthen this by adding a question at Registration.
http://www.vbulletin.com/forum/showthread.php?t=205214

These thing only help to deter spammers, they do not stop them.
Applying the above will almost certainly reduce the amount of spammers, however Moderators who quickly delete spam posts will send a message to all.

After applying above, you can find MOD's that restrict members from posting links and the likes in the MOD section.