Invisible Captcha

comment spam bots love form fields

When they encounter a form field

fill out each and every field

spam bots tend to ignore CSS

if you use CSS to hide a form field

they have a really hard time knowing that the field is not supposed to be visible.

you can create a honeypot form field that should be left blank and then use CSS to hide it from human users

When the form is submitted, you check to make sure the value of that form field is blank

if(!String.IsNullOrEmpty(Request.Form["honeypot"]))  IgnoreComment();