Good Question. There clearly should be a means of deleting unwanted ones within the interface without having to hack the program.
They are stored in the User/YourName/AppData/Local/Pinnacle_Studio_21/Studio/21.0/Export folder, where there is an xml file for each preset that has been customized. If you delete the XML then you revert back the original presets. I'd suggest caution here - there are XML files for a lot of the Devices and others that I'm not sure off. If you are happy to edit the XML files in Notepad then you could just remove the entry you want to.