Now a days smartphone and tablets provides you a slider for unlocking your device. You might have seen this feature in iPhone, Nokia N-Series, Sony Xperia and other popular brands; their smartphone automatically locks and to unlock it you need sliding your finger from left to right on the screen. Now you can use same kind of slider in your WordPress site for fighting spams in comments.
Install and activate Slide2Comment; after activation this plugin automatically hides your comment form and adds a slider in place of it, now for dropping comments your users have to slide their mouse from left to right which opens up the default WordPress comment form.
There are no more settings; simply activate and it immediately starts working. The plugin allows you to edit its CSS (Dashboard -> Settings -> Slide2Comment) and also shows preview of edits you are making.