Just out of curiosity, is adding nousb to the grub command line actually feasible for enforcement?
I can't remember the last time I used a system where I didn't need a USB keyboard at some point (can you even buy server class systems with PS/2 support any more?)
WARNING: Disabling all kernel support for USB will cause problems for systems with USB-based keyboards, mice, or printers. This configuration is infeasible for systems which require USB devices, which is common.