How to disable on board bluetooth on RPI4 and not the bluetooth usb

[kalpana]

Hello,
I searched the internet for a solution, but couldn't find a suitable one. I blocked the UART (ttyAMA0) and tried other suggestions, both the on board and the usb BT are disabled then. How can I disable the on board BT and have the usb BT enabled. I use a metal box and the bluetooth is weak.
Thanks in advance
Kees