Intel put out some BIOS releases that provided a parameter that allowed the onscreen POST code display to be enabled/disabled. Whether they did it for this board I am not sure. Go into BIOS Setup and look to see if it is there (sorry, I no longer have access to one of these boards to check myself)...
If there is an option I'm just not seeing it. I double checked the BIOS version and it is version 650 which was the last one available. I'll keep digging around. Maybe there is another way to set that BIOS option.
I went ahead and pulled down the Intel Integrator Toolkit in order to see if there was something that could be customized in the BIOS display options but there isn't anything about POST code enable/disable. I can disable displaying POST function keys but not the actual codes.