|Firmware versions 122 onwards have an option for this - see C2-4000 & C2-5000 series firmware updates.|
Go into 'Setup Program source' and change the input standard from 'PAL / NTSC' to 'B&W 50Hz / 60Hz' (it's the last option).
Alternatively, this can be done by feeding the signal into the Y BNC connector on RGB3, and setting RGB3 as 'YUV' in the 'Setup Program source' menu. The U & V will not be used, hence no colour will be displayed.
(Or similarly feed the signal into pin 2 of the RGB1 or RGB2 connectors, using pin 7 as ground).