I don't know what AM2R is, but if it doesn't support blutooth/usb controllers by default then I guess it also doesn't allow you to remove the controls.
You can map an external controller to mimic touch-screen events by using something like
.
Removing the onscreen controls is more tricky and depends on how they've been coded. If they've been coded using graphical resources, then you could try using Apk Tool to decompile and recompile the binary, replacing the appropriate resources with blank transparent image files as you go. If they draw the controls by code, then there's nothing you can do, unfortunately, other than ask for a refund and post negative feedback for the devs being lazy shits and not taking the time to implement support for external controllers.