Wagner

<ul>
<li>Block certain kinds of users from registering to your WordPress site by adding blacklist rules. </li>
<li>Super simple to use. The whole plugin just has one simple settings page.</li>
<li>Support regular expression rules.</li>
<li>Completely free.</li>
<li>Support tracking and recording login attempts since v1.1.0.</li>
<li>Login attempts chart since v1.1.0.</li>
</ul>


Geek Mail Blacklist

Block users with certain emails from registering to your WordPress site by adding blacklist rules.

Kat Hagan

<p>This plugin allows you to selectively disable comments for visitors from certain sites (e.g. reddit).  If a visitor’s HTTP referer matches one of the hosts you’ve blacklisted, your post will look as if comments have been disabled (neither existing comments nor the new comment form will be shown).</p>


Disable Comments By Referer

Ban visitors from certain sites from viewing or adding comments.

Geek Mail Blacklist vs Disable Comments By Referer