- removing links (making them as plain text forcing users to copy paste them..) - hiding links from non-registered users (plain text to non-registered users, clickable for registered users), - blocking links from search engine crawlers (robots.txt / rel=nofollow...).
Maybe these fall in the "for each complex problem there is simple but wrong solution" but it sounds like it's worth a try.
A service like Stop Forum Spam might be a solution to this. It checks for IP address and email address and gives it a value based on how likely it is assumed to be a spammer.
When they have to set up a new email account and maybe even a new IP address for every few accounts, it gets to be a lot of work soon.
https://www.stopforumspam.com/
It has a very simple API and is not that hard to implement (really, I have done it myself :) )
https://plugins.trac.wordpress.org/browser/gwolle-gb/trunk/f...
That function can be called when the register form has been submitted. It will return true or false. Forget about the transient stuff, that is just WordPres caching stuff.
You don't need an API key like with Akismet. You would only need it if you want to add or remove entries from the SFS database. It really is much simpler. Ofcourse you might want to have a checkbox in the settings. But still, in an afternoon you might be able to finish this :)
Wish you the best.