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.