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.

termel

Simply prevent duplicate submissions by blocking the submit button while submitting

Gravity Forms Prevent Duplicates

Does the job

Simply prevent duplicate submissions by blocking the submit button while submitting

Spam Prevention for Contact Form 7 and Comments vs Gravity Forms Prevent Duplicates