Hi DerRazputin,
The ART tools are scripts, not a plugin, which is why you can’t load/unload them from the Plugin Manager. The userSetup.py adds a ScriptJob to the current session which runs the mayaTools() method whenever a new scene is opened. You could try commenting out the scriptjob setup (last line in the file), then create a Python shelf button that runs that method manually. This would allow you to only load the tools on command. However, since the tools are Python scripts, though, once they’re loaded the only way (AFAIK) to unload them is to restart Maya.