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I think that one option is to only use a captcha if the edit includes external links. I agree that random captchas or on every edit would only piss off the regular users.I do like the idea of the question about the game created by us, rather than random words or math problems. You could create something blindingly obvious to all BattleMaster players that a non-player would not really be able to answer.
I may be jinxing things here, but this appears to have worked. We have not had a case of spamming in the past 7 days.