Yevhen Horbyshyn

This plugin adds a hidden “inverted” checkbox to your forms to prevent bots from submitting them.
<ul>
<li>Adds support for Contact Form 7 via shortcode <code>[ica_checkbox]</code></li>
<li>Adds JavaScript-based hidden checkbox to regular forms in the footer</li>
<li>Blocks form submission if the checkbox is unchecked or missing (typical behavior for bots)</li>
</ul>

Inverted Checkbox Antispam

Simple spam protection using an inverted checkbox. Compatible with Contact Form 7 and custom HTML forms.

Fabio Santos

This plugin adds a simple checkbox in the insert/edit link popup for including nofollow attribute.

Rel Nofollow Checkbox

Adds a checkbox in the insert/edit link popup for including rel="nofollow".

Inverted Checkbox Antispam vs Rel Nofollow Checkbox