1. Check for an uninstaller — If the plugin has administration pages, check them to see if the plugin has an uninstallation routine. If so, running it should clear out the plugin’s database tables and settings, which you won’t be needing anymore if you don’t plan on using the plugin again.

2. Deactivate the plugin — Go to the “Plugins” section, find the plugin, and then click “Deactivate.”

3. Check for plugin calls — Now browse around your site and see if anything broke. If so, follow the instructions in this post: What to Do if Plugin Deactivation Breaks Your Blog.

4. Delete the plugin files — Using an FTP program, delete the plugin’s folder and/or file.

You may also want to check out the Clean Options plugin. I can’t vouch for it, since I haven’t used it, but it claims to be able to find any options table entries that the plugin may have left behind.

The problem is that your theme is calling on the plugin you deactivated. However, since that plugin is gone, the site displays an error and then stops rendering the rest of the page.

Here’s what to do:

1. Go to your WordPress administration. Click “Design” (or “Presentation”), then click “Theme Editor”.

2. Where on your site did the error appear? If it was on your sidebar, click “Sidebar.” If it was only on a search page, click “Search Template.” If it was in the comments section, click “Comments,” etc.

3. Now look for a function call that might be related to the plugin you just deactivated. For example, if you deactivated the Related Posts plugin, you might see some code like this:
   <?php related_posts(); ?>

4. If you don’t plan on using the plugin again, and if you’re sure the code belongs to the deactivated plugin, just delete the code.

   Or instead, you can follow these instructions to alter the plugin function call so that it won’t break your blog when the plugin is deactivated.

   (If you’re new to theme editing, you might want to backup your theme first just in case.)

5. When you’re done, click the “Save” button to save your changes.

6. If you couldn’t find any relevant code, or if the problem still isn’t fixed, repeat steps 3-5 for the rest of your theme files.

If all else fails, reactivate the plugin to get rid of the error, and then consult the plugin’s documentation, contact the plugin author, or file a support request at the WordPress Support Forums.