As mentioned above in the thread, multiple times, maybe a simpler solution to reduce spam is to remove incentives by:

- 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.