Radek Mezulanik

This plugin:
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Add 3 hidden fields to Registration form and to Comments form, that only robots can see. Plus you can choose to add validation checkbox to forms.
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Add to .htaccess file direct access protection for comments file.
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Check time user spent on page and time when user send a form.
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Disable trackbacks for past and future posts.
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Additionally you can choose to add validation checkbox to forms.

Antispam Revolution

Three way antispam protection for registration and comment forms.

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.

Antispam Revolution vs Spam Prevention for Contact Form 7 and Comments