how to turn off the serial port and Arduino device on Pro Micro?


I have troubles searching this out, so hopefully someone will point me to a better direction.

I want to use my Pro Micro (32U4) as a pure keyboard device. Is there a way to disable the Arduino Device and Serial port? I want to disable or hide all devices except the HID one. I will need to program the board via ISP header of course

I assume it can be done by writing an alternative bootloader, but I need some advices how to do it, or how to find out how to do it

thank you