I've deployed Google's "I am not a robot" captcha on over 30 websites and found it to be very effective.
It also addresses the cat-and-mouse situation where the webmaster is constantly having to deploy new methods to keep the bots out. Google keeps the process updated so they take care of that.
Maybe people who sign and post while having fewer than 5 posts have to tick the 'I am not a robot' box.
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