Serkan Algur

You can set a search limit for visitors via IP addresses. The plugin automatically detects visitor IP address and counts each search action of these IP addresses. You can set a limit and block time for these IP Addresses.

Search Limiter & Blocker

Set and limit search count of visitors. Also, you can block visitor IP.

Ryan Welcher

Patrick Rauland

This is a simple WordPress plugin which allows a site adminstrator to limit the number of widgets in a sidebar. You simply go the the settings -> limit widgets page and customize the maximum number of widgets used for each menu.
A nice visual effect will appear in the sidebar in the WordPress admin and the client will be unable to drop new widgets into the sidebar once the cap has been reached.
You can set widgets to have unlimited widgets by leaving the field blank. You can also set the sidebar to have 0 widgets effectively diabling it.
Credits
This is the Internet which means that nothing is created from scratch. This plugin is mostly a combination of work from other people which I just packaged into something with a user interface for easy management.
<ul>
<li><a href="http://wordpress.stackexchange.com/users/8/jan-fabry" rel="nofollow ugc">Jan Fabray</a> on <a href="http://wordpress.stackexchange.com/questions/19907/limit-number-of-widgets-in-sidebars" rel="nofollow ugc">WordPress Answers</a></li>
<li><a href="http://stackoverflow.com/users/821185/aman" rel="nofollow ugc">Aman</a> on <a href="http://stackoverflow.com/questions/6636701/limit-number-of-items-in-sortable-with-draggable-as-source/6663581#6663581" rel="nofollow ugc">Stack Overflow</a></li>
</ul>
Contributors Welcome
<ul>
<li>Submit a <a href="https://github.com/BFTrick/limit-widgets" rel="nofollow ugc">pull request on Github</a></li>
</ul>
Author
<ul>
<li><a href="http://www.patrickrauland.com" rel="nofollow ugc">Patrick Rauland</a></li>
<li><a href="http://www.ryanwelcher.com" rel="nofollow ugc">Ryan Welcher</a></li>
</ul>

Limit Widgets

A plugin for WordPress that limits the number of widgets in a sidebar.

Search Limiter & Blocker vs Limit Widgets