This was a very specific vBulletin thread about preventing spambots that was
very easy to find and was posted almost five years ago.
https://www.vbulletin.com/forum/show...TOP-Spam-Bots-!!
The first post explains and links to a plugin that can be installed that prevents users from posting links until they reach a certain number of posts (which can be manually set).
Apparently this is very effective, and even though spambots still register they're completely unable to post.
SOUNDS GOOD TO ME, LET'S GET THIS SHIT SET UP ASAP