The USART, which provides TX and RX functionality, is disabled by default. However, the serial bootloader that we provide for the A-Star 328PB enables it in some circumstances before your program runs. If you program using the ISP interface, you would erase the bootloader, meaning that the USART would always be disabled by default.
If you want to use the bootloader instead of ISP, you should be able to disable the USART at the beginning of your program with this line of C/C++ code:
UCSR0B = 0;
–David