Hi Stephan,
Martin has best solution to disable the default io capability.
Another method would be to make modifications to the user_config.h (this is the highest level header file in the RTOS that can over ride default RTOS settings).
Simply setting all the selected BSPCFG_ENABLE(I)TTYx #defines to 0 will disable the RTOS from using a physical UART.
That doesn't resolve the printf's matter you have in your code base.
For that you can modify the PSP fio.h header and add a #define to the user_config.h to switch on and off the printf capability:
fio.h modifications:

user_config.h modification:

As always, please re-compile the RTOS when making modifications to user_config.h.
Regards,
David