Welcome to Android Central! Stock ringtones should be located in the directory /system/media/audio/ringtones. You'll need a 3rd party file manager to access that directory, since the stock file manager won't allow you to go there. Try Total Commander, and select "File system root" then look for the above directory. I'm not sure if it will let you delete any of the ringtones (I didn't try it myself)--if you're able to, report back so that others can know!