I don't think you can. I tried for a long time to find a way to get the wireless mouse I use to not wake the computer when I bumped the mouse, and never found a way. You can disable bluetooth devices from waking the computer, but not wireless.
I do not believe there is a way. Macs and many newer PCs are set at the Firmware/BIOS level to wake on USB activity.
Though there may be a way to change this in the Mac Pro's firmware this would likely also make it impossible to wake you Mac via a mouse click or pressing a key on your keyboard.
Often on PCs that you build yourself the USB wake option is in the BIOS or a jumper setting on the motherboard. This option is not always made available on manufactured computers since they neuter the BIOS to keep buyers from screwing up their computer.