You could set Calibre to IGNORE the device (can be done when attached),
BUT
that is rather severe, if you want to later use Calibre with that device
If Calibre is not running, there is no prob.
If Calibre is running, simply use Eject device on the Device Icon. (I think Calibre ignore a second devices attempt to connect, if another is in use)

PC USB ports are current limited (500mA). A real charger is faster and almost required for many Tablet style devices.