Only one remark: What will you do when spammers train their bots to make automated constructive and helpful comments?
Otherwise, keep up the great work!
Seriously: Directly, actively preventing spam from being posted in the first place (error: unable to post this, because of spam) might cause spammers to quicker work around the system. One should assume that spammers feel less motivated working around a prevention system, when they actually still think, they get their messages delivered. Therefore, I like this pragmatic and successful approach!