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.

Taras Kushnir

Private Captcha WordPress Plugin integrates Private Captcha with your WordPress site to protect forms from spam and abuse.
Features: 
* Protect login, registration, password reset, and comment forms 
* Flexible widget configuration (theme, language, start mode) 
* EU compliance with EU-only endpoints and custom domains 
* WP-CLI commands for emergency management 
* No tracking, privacy-focused captcha solution
Supported Forms: 
* WordPress (Login, Registrations, Password reset, Comment forms for logged-in/guests) 
* WPForms 
* Contact Form 7 
* Gravity Forms 
* Elementor Pro 
* WooCommerce (Login, Registration, Password reset, Checkout guest/logged-in) 
* Fluent Forms 
* Forminator 
* Formidable 
* Ultimate Member (Login, Registration, Password reset) 
* (more forms support is currently in progress)
More forms (including popular plugins) are currently in progress of being added.
<h3>External services</h3>
This plugin uses <a href="https://privatecaptcha.com" rel="nofollow ugc">Private Captcha</a> API to request and verify captcha challenges and their solutions, when protection is enabled on user-configured forms.
When solving of the captcha widget challenge begins, a new challenge is requested from Private Captcha API. When submitting the relevant form (e.g. Comment Form or Login Form), challenge solution is sent to Private Captcha API.
During requesting challenges or verifying solutions, no Personal Data (within meaning of Art. 4 (1) GDPR) is neither collected, nor sent. Check all relevant information in the Private Captcha’s <a href="https://privatecaptcha.com/legal/privacy/" rel="nofollow ugc">Privacy Policy</a>.

Private Captcha

Private Captcha protects your WordPress website from spam and abuse with a privacy-first, independent CAPTCHA solution made in EU.

Antispam Revolution vs Private Captcha