Kasuga

Tiny Comment Spam Blocker is a lightweight yet powerful plugin designed to protect your WordPress comments from spam. It employs five different techniques to detect and block unwanted comments:
<ol>
<li>Nonce Verification – Ensures that the comment form submission is genuine.</li>
<li>Submission Time Check – Blocks comments submitted too quickly to prevent bots.</li>
<li>Honeypot Field – Hidden field that traps automated spam bots.</li>
<li>User Agent Validation – Detects suspicious User-Agent strings and blocks them.</li>
<li>Forbidden Word Filtering – Blocks submissions containing words or phrases from a configurable list within the comment body, email address, or IP address.</li>
</ol>
These filters are applied in order: if a comment passes the first check, it proceeds to the second, and so on, until all checks are applied or the comment is blocked.
Additional features:
<ul>
<li>Option to log detected spam in a local log file (up to 1.0 MB).</li>
<li>Optional email notifications when spam is detected.</li>
<li>Easy settings page in the WordPress admin panel.</li>
</ul>
<h3>Settings</h3>
The plugin provides the following settings in the WordPress admin panel:
<ul>
<li>Enable Spam Protection – Toggle the spam protection on or off. When disabled, all anti-spam checks are skipped.</li>
<li>Save Spam Detection Log – Enable or disable logging of detected spam. Logs are saved in a local file up to 1.0 MB within the WordPress uploads directory.</li>
<li>Notification Email Address – Enter an email address to receive notifications when spam is detected. Leave blank to disable email notifications.</li>
<li>Minimum Submission Time (seconds) – Set the minimum allowed time between loading the comment form and submitting a comment. Comments submitted faster than this threshold are considered spam.</li>
<li>Forbidden Words List – Enter one forbidden word, phrase, or IP address per line. Submissions containing these entries in the comment body, email address, or IP address will be blocked. Case is insensitive.
<ul>
<li>Example: 
viagra 
online pharmacy 
spam@email.com 
164.138.205.72</li>
</ul>
</li>
</ul>
<h3>Arbitrary Section</h3>
This plugin is designed to be lightweight and fast, ensuring minimal impact on site performance while providing robust protection against comment spam.

Tiny Comment Spam Blocker

A simple and lightweight yet rock-solid plugin that blocks comment spam using multiple automatic detection methods.

WP-Biz (Ryan)

The “honeypot” technique for SPAM protection is invisible to humans, and tricks robots into filling out an invisible form field. Then, if that invisible field has been filled, the form cannot be submitted. Easy, non-instrusive SPAM protection.
No settings to set, this honeypot is added to all Formidable forms on your site, unless the user is logged in. When SPAM is suspected, the form will not be submitted and the visitor will be alerted to this fact.
Tested to work with both Formidable (free) and <a href="http://www.wpbiz.co/go/forms" rel="nofollow ugc">Formidable Pro</a> (best bet for awesome form power!)
A note to Formidable Pro users: You paid to receive extra features in the Formidable plugin. Me too, it’s well worth it. This Honeypot plugin, and Ryan (the developer) are not connected to Formidable in any way. Since this adds another very helpful feature to Formidable, I ask you to send a <a href="http://wpbiz.co/donate/" rel="nofollow ugc">donation at WPbiz.co</a> to say thanks and encourage future developemnt.

Formidable Honeypot

Add SPAM protection to your Formidable forms without annoying your visitors with Captchas, math, or other fields.

Tiny Comment Spam Blocker vs Formidable Honeypot