The spam problem has persisted despite me adding a maths-problem captcha and a logic-problem captcha. This either means people have found a way to bypass this page completely, or people are actually being paid to go through the signup process themselves and post the messages manually. This has forced me to take further action. If your account is under a certain age, you will not be able to make a post with a URL in it.

We'll see how this solution works.