To clarify, are these actual plug-ins, or FCP Effects created in Motion?
If you are talking about actual plug-ins the best thing to do is run unistallers that may be added to your system when you originally installed the demo. For example FXFactory and Boris install ways to do this when you add the plug-ins to your system. Look for "Uninstaller" packages in the Application folder. Otherwise, check the website you got them from, usually they will have instructions on how to remove plug-ins, or you can contact them directly for instructions.
If these are FCP Effects, they should be in your ~/Movies/Motion Templates/.... Some of these are actually a combination of 3rd party plugins that then include templates (like the ones mentioned above). Effects in this folder can just be dragged to the trash and deleted, just make sure you don't delete ones that you need. If you are more specific about who's plug-in/effect you are trying to remove it would be good to include that information in your post.