That might help for sites with a small userbase. However these are large platforms with large clusters of users that can be easily contacted. They'll just invest in a captcha cracking service which runs at about a 1000 solved captchas for $3. If you ban datacenter ips, use dnsbl and scan for proxies, they'll switch to rented dsl lines.
On the other end are the users. If you ban proxies, finger their ports and ask them to solve a captcha every time they hit the submit button; you'll create some serious animosity. Stopping spam means having to invest, come up with complicated algorithms and you still might accidentally ban innocent users who will blog about this or tell their friends .
The real question is... does it matter that affiliate links are being posted if it needs a guide to let the everyday users notice it . My niece doesn't even know what a affiliate link is and neither do most users. I mean if there's a 100% method to stop it, implement it. However, should you invest money and dev time into problems that nobody has solved to this date...
manual captcha breaking services are available for as low as $10 for 1000 captcha breaks. Most of the bots use them, just pass the captcha image to the service and there is someone at the other end who would just enter the characters and all of these happens within seconds. search deathbycaptcha etc.