Stop Android connecting to external USB storage device
Use a needle or a pair of needle-nose pliers to break the data pins on your adapter, so they won't connect to the device. Breaking the data pins doesn't inhibit the adapter's ability to supply voltage across the power pins. Alternatively, it might be easier to cover them with a small piece of insulating tape; the easiest way to do it will depend on the shape of the adapter and what tools you have to hand.

Of the four pins inside a USB plug/socket, the middle two are the ones you want to cut off or cover. Be careful not to damage or cover the two outer pins. It should go without saying that you should do this while the adapter is not connected to anything.