Look in the nbs_groupspawner.sqs for this section:
; Defining Enemy Class
~0.5
? (_enemyside == EAST): _enemy = ["SoldierEB", "BMP2", "T72", "ZSU", "URAL", "BRDM2", "UAZMG", "KA50", "MI17_MG", "SU34"]
? (_enemyside == WEST): _enemy = ["SoldierWB", "Truck5tMG", "HMMWV50", "StrykerBase", "M1Abrams", "M113", "AH1W", "UH60MG", "AH6", "AV8B", "Ship"]
? (_enemyside == GUER): _enemy = ["SoldierGB", "AH6_RACS", "MH6_RACS", "Vulcan_RACS", "M113_RACS"]
; --------------------------------------------------------------------------
Now, if you won't the groups to be created if you fly with a blackhawk over the area, just delete the corresponding class, in this case "UH60MG". This also affects the UH60 with FFAR as it is a subclass of the UH60MG.
I suggest you make these changes just in the mission you're actually making and keep the "original" files untouched.