georf

This plugin uses JavaScript to verify that commentators are persons rather than bots.
If a user’s browser doesn’t support or allow JavaScript, so-called CAPTCHAs are used. The commentator has to enter a fruit name given as image before sending his or her comment.

Obstcha

Decent comment verification to avoid spam without user interaction.

webvitaly

kudlav

This is fork of successful Anti-spam plugin v5.5 written by webvitalii, for more info visit <a href="https://github.com/kudlav/anti-spam/" rel="nofollow ugc">GitHub Fork</a>. 
From version 5.6 maintained by kudlav.
<ul>
<li><a href="https://github.com/kudlav/anti-spam/" rel="nofollow ugc">GitHub</a></li>
</ul>
Anti-spam Reloaded plugin blocks 100% of automatic spam messages in comments section and also blocks all trackbacks.No captcha required.
Plugin is simple and easy to use: just install it and it just works.
Blocked comments can be stored in the Spam area and converted to regular comments if needed.
Anti-spam Reloaded plugin is GDPR compliant and does not store any other user data except of the behaviour mentioned above.
Plugin blocks spam only in comments section.. 
Plugin does not block manual spam (submitted by spammers manually via browser).
<h3>Compatibility</h3>
All modern browsers and IE11+ are supported. 
Anti-spam Reloaded plugin works with disabled JavaScript. Users with disabled JavaScript should manually fill current year before submitting the comment.
Server compatibility:
<ul>
<li>WordPress 3.3 – 6.5</li>
<li>PHP 5.6 – 8.2</li>
<li>Doesn’t use jQuery</li>
</ul>
Plugin is incompatible with:
<ul>
<li>Disqus</li>
<li>Jetpack Comments</li>
<li>AJAX Comment Form</li>
<li>bbPress</li>
</ul>
If site has caching plugin enabled and cache is not cleared or if theme does not use ‘comment_form’ action 
and there is no plugin inputs in comments form – plugin tries to add hidden fields automatically using JavaScript.
<h3>How does it work?</h3>
The blocking algorithm is based on 2 methods: ‘invisible js-captcha’ and ‘invisible input trap’ (aka honeypot technique).
<h4>‘invisible js-captcha’</h4>
The ‘invisible js-captcha’ method is based on fact that bots does not have JavaScript on their user-agents. 
Extra hidden field is added to comments form. 
It is the question about the current year. 
If the user visits site, than this field is answered automatically with JavaScript, is hidden by JavaScript and CSS and invisible for the user. 
If the spammer will fill year-field incorrectly – the comment will be blocked because it is spam.
<h4>‘invisible input trap’</h4>
The ‘invisible input trap’ method is based on fact that almost all the bots will fill inputs with name ’email’ or ‘url’. 
Extra hidden field is added to comments form. 
This field is hidden for the user and user will not fill it. 
But this field is visible for the spammer. 
If the spammer will fill this trap-field with anything – the comment will be blocked because it is spam.

Obstcha vs Anti-spam Reloaded