Simply block the link posting of new members until they show themselves to not be spambots after which point, they are granted the privilege?
There are certain IP ranges (such as the Ukraine) which are rarely going to be getting actual members registering, yet there is always the chance that legitimate users are affected, and I'm certain if that's the case, you could always set up an email address which they could contact to get their IP whitelisted. If it turns out they are a spambot, back on the blacklist.
Could potentially tweak the custom questions a little and see if it works.
On a side note, if the same links keep getting spammed, just blacklist the URLs so they can't be posted.