This is a way to cut comment spam without using captcha

Dec 2, 2013 at 3:32 AM
There's a simple yet non-friction way of cutting most comment spam generated by spam bots, and BlogEngine should implement it by default.

On the post/comment page add a dummy text field and hide it using css or javascript. An automated bot will automatically fill it. In code do a quick check. If the text field is filled you know a bot filled it and the code ignores the comment.
There might be smart bots which can 'see' fields which can defeat this trick but this simple method costs nothing and is zero friction for the user.