So this is Bukkit plugins? The general rule for the plugins is that they each have a folder that is their name. Two plugins will not use the same folder, unless they have the same name, which is highly unlikely, and I don't even think it is possible. So, by simply deleting the plugin .jar file and the folder with the same name, you will have removed the plugin.
If you cannot find a folder with the same name as your plugin, look for one with a similar name. If you are still unable to find the folder for it, look at fredley's suggestion: try creating a new Bukkit server on your computer, add the plugin in question, and find what files and folders it creates. These are the files that you want to delete. If you accidentally delete a folder, just restore it from the Recycle Bin or Trash.
If you still cannot figure out which folder to delete, just don't delete any. This will cause no harm, all of the plugins will work as they used to, and there will be no risk of accidentally deleting a folder that is needed for another plugin.