In my opinion —
Plugins like this, and those of MotionVFX, FxFactory and Pixel Films Studios as essentially viruses you download and install. They all have "manager" software used to control the state of the plugins (applying watermarks, etc.) and for all I know, they can watch what you do on your computer. I will grant you that this system of software management is "sanctioned" by Apple, but there are still no details provided on exactly what their software is doing when you install it. One thing it does do is reinstall a plugin any time you delete it from your hard drive...
[my 2¢]
When Final Cut Pro X was released, the template/plugin installation was specifically recommended that they be installed in your User home folder in a specific path. Although multiple users can use any given machine, the plugin installation was designed to be solely for a single user. There is, however, another location for "all users".
This particular plugin (Nitrate) has a manager. From what I can tell, it is located in:
/Library/OFX/Plugins. The plugin itself might be called FilmConvertPro2.0.ofx.bundle
And there is another! (what a surprise):
/Library/Application Support/RubberMonkey
After this, you will have to track down the plugin FCP uses in your projects. I *believe* it is in the "all users" location of:
/Library/Application Support/Final Cut Pro/Templates/Effects/... then whatever category they force on you when you installed the demo.
You should probably also check to make sure it is not in:
/Users/<home folder>/Movies/Motion Templates/Effects/...
Once you get rid of the "manager" software, you should be able to easily, and permanently delete the individual templates.
HTH