Brandon Fenning

Since the vast majority of WordPress spam is automated in nature, the goal of BotBlocker is to block comment spam bots and not hinder users. This is accomplished through the use of honeypot technology, which tricks the spambot into identifying itself by performing actions that a human could not.
BotBlocker’s features include:
<ol>
<li>Works out of the box to block spam</li>
<li>Automatically stops spam-bots from commenting</li>
<li>Zero hindrance to users: no CAPTCHAS, math questions, passwords or extra input required</li>
<li>Zero false positives and ignores registered users</li>
<li>Can be configured to completely block spam-bots or flag comments as spam</li>
<li>Spam detection messages can be easily adjusted</li>
<li>No javascript or cookies required</li>
<li>Hooks into <code>wp-comments-post.php</code> to prevent spam-bots from directly submitting spam comments</li>
<li>Should work fine with most customized comment forms</li>
<li>No API keys required</li>
</ol>
http://www.lform.com/botblocker/ for more information.
Plugin by <a href="http://www.lform.com/" rel="nofollow ugc">LFORM, a web design company</a> in New Jersey.

BotBlocker

Kills spam-bots, leaves humans standing. No CAPTCHAS, no math questions, no passwords, just spam blocking that stops spam-bots dead in their tracks.

Per Søderlind

Based on a users IP address, WP-DenyHost will block a spammer if he already has been tagged as a spammer. Use it together with the Akismet plugin. Akismet tags the spammer, and WP-DenyHost prevents him from accessing you site.
If you have a <a href="https://www.cloudflare.com" rel="nofollow ugc">CloudFlare</a> account, the plugin can add spamers to <a href="https://www.cloudflare.com/threat-control" rel="nofollow ugc">CloudFlare Block list</a>

WP-DenyHost

WP-DenyHost denies a spammer from accessing your WordPress site. WP-DenyHost can add spammers to the CloudFlare Block list

BotBlocker vs WP-DenyHost