I just whipped this up:
I’ve been struck by comment spam in the last few days with several spam comments awaiting moderation before I turned on akismet to block them.
This morning however, after 54 blocked comments, one got through akismet and I decided to try my hand at a WP plugin.
It turned out to be surprisingly easy, perhaps only 20 minutes work, including the learning of how wp plugins work for comments.
Credits to jroller for the inspiration. I guess we’ll find out if a spammer is interested in beating it - I’ve several ideas for improvements, but I’d rather keep them in reserve for when it gets beaten (which could be an hour from now, or a year 
)
