Why can't we just use CAPTCHA? I don't see why people should need to know any Lemmings-related trivia to sign up. I was reeled into the forum by Lix, not Lemmings, though I did play Lemmings a long time ago. I can imagine there might be users who would be interested only in some of the more tangential topics, and not care about Lemmings in the slightest. Sometimes users bring in their friends for Family Feud, for example, and they might not know anything about Lemmings at all.
Asking Lemmings trivia is just irritating, and those sorts of security questions always struck me as unprofessional anyway. On the other hand, CAPTCHA is expected and common to most forums.