Have you tried to turn off the speaker and then when turning it back on pair your device so the other one can't come in?
DM-40BT Disable Bluetooth
How do I disable the Bluetooth on this speaker? I only used the wired connection and someone else has connected to it and there is no way to kick them off. I've tried pairing other devices, cycling power, etc. The manual is no help either.
I can play over the wired connection but it does not override the bluetooth. It plays both inputs.