sdesalas

This plugin allows WordPress to block over 95% of spam comments using a time-based hash.
This allows for a minimum sanity check and yet should remove almost all spam comments 
without the need to sign up to any third party APIs.
The plugin has been tested in real world conditions for around 12 months. It will not block 
real comments and it is pretty effective against spam, however it only uses a very simple 
algorithm and for that reason some spam comments may still make it through.

WP Simple SpamCheck

This plugin allows WordPress to block over 95% of spam comments using a time-based hash.

ctomczyk

Spam Prevention for WP Contact Form 7 and WordPress Comments. 
This plugin adds a small code snippet to determine page scroll and add dynamically a unique token. 
It also supports the “Contact Form 7 Database Addon – CFDB7” and prevents contact form submissions from being saved if validation fails.
If the token doesn’t exists while submitting or call directly API then the incoming request will be rejected.
You can see how it works on <a href="https://www.sitelint.com/contact" rel="nofollow ugc">SiteLint</a> website.

Spam Prevention for Contact Form 7 and Comments

Spam Prevention for WP Contact Form 7 (manage multiple contact forms) and WordPress Comments.

WP Simple SpamCheck vs Spam Prevention for Contact Form 7 and Comments