I was still having this issue in Windows 10 three years after you posted it. My solution was to go to Settings > Devices > Typing and uncheck "Show the touch keyboard ...when not in tablet mode and there's no keyboard attached." Then, I put my tablet into desktop mode when my keyboard is connected and back into tablet mode when it's not. In desktop mode the touch keyboard doesn't appear; in tablet mode it does.
This isn't an automatic solution - ideally the touch keyboard would know not to appear when a Bluetooth keyboard is connected regardless of whether the tablet is in touch or desktop mode - but at least this way I can control the behavior of the touch keyboard by switching modes.