Radek Mezulanik

This plugin:
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<li>
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.
</li>
<li>
Add to .htaccess file direct access protection for comments file.
</li>
<li>
Check time user spent on page and time when user send a form.
</li>
<li>
Disable trackbacks for past and future posts.
</li>
</ol>
Additionally you can choose to add validation checkbox to forms.

Antispam Revolution

Three way antispam protection for registration and comment forms.

Arnan de Gans

Tired of spam bots in your WordPress and ClassicPress website? Do you want to get rid of false registrations and other spammy nonsense? Don’t wan’t to use a clumsy and user-unfriendly Captcha? Don’t want to use a Captcha from Google or other big-tech company period?
Meet No-Bot Registration, easy to use, superior protection without making it hard for your visitors. Easily blacklist (partial) email addresses and domains so they can no longer register an account.
Create one or more questions and a set of possible answers for them and visitors have to answer your question when they register. 
If they answer wrong, they get denied their account.
Questions can be as simple as “1 + 1”, with possible answers being 1, one or uno. That way you can plan for eventualities and how people interpret your question.
<h4>Features</h4>
<ul>
<li>Protect registration forms</li>
<li>Protect the WooCommerce checkout form if you let people register from there</li>
<li>Protect your blog comment form</li>
<li>Prevents comment spam, trackback spam and other nuisances with ease</li>
<li>Set up multiple security questions to further confuse bots</li>
<li>Blacklist any email, domain or tld you don’t like</li>
<li>Configurable notification messages for users failing the security tests</li>
</ul>

No-Bot Registration

Prevent bots from creating accounts by blacklisting domains and usernames and present people with a human friendly security question.

Antispam Revolution vs No-Bot Registration