Ok, I've figured it out. All you need to do is go to %AppData%\calibre and edit the file mtp_devices.json using any text editor. You should add your device code to blacklist; for example:
Code:
{
"blacklist": [
"R51F601VHWF"
],
"history": {
"R51F601VHWF": [
"victorvscn (Potato phone)",
"2015-07-03T21:13:35.328000+00:00"
]
}
}
My device code was R51F601VHWF, since it corresponded to the entry under history, so I added that code to the blacklist. If your blacklist already has a device, then you have to use a comma to separate your new comma (and, obviously, use the double quotes).