Delete its folders in the file system. I don't think there is a better way...
The soul is dyed the color of its thoughts. Think only on those things that are in line with your principles and can bear the light of day. The content of your character is your choice. Day by day, what you do is who you become. Your integrity is your destiny - it is the light that guides your way. - Heraclitus
Gabor, A few plugins offer an option in the add/remove programs of Windows. These are generally the ones that come with a full installer.
The plugins that you just unzip into the directory need to be deleted by hand as Ilja said. If you have access to a copy of the zipped version of the plugin, you can see what folders it created that way.