You may define actions for keys in ArmA, as far as I know, no way in OFP. But you can define menu actions. In your mission, do the following:
Create an init.sqs script if still not present:
;init.sqs
stopsmoke = true
;Change _relpos to have the smoke comming out from the desired vehicle spot
_relpos = [0,0,0]
[yourvehiclename, _relpos]exec"whitesmoke.sqs"
player addAction ["Switch smoke", "switchsmoke.sqs"]
exit
And the switchsmoke.sqs script
;switchsmoke.sqs
stopsmoke = !stopsmoke
And finaly change your smokescript this way (lets say it is named whitesmoke.sqs)
;whitesmoke.sqs
;Execute it passing vehicle and vehicle's relative position of the smoke
_vehicle = _this select 0
_relpos = _this select 1
stopsmoke = true
_size = [1,2.5,3,3.5]
_colours = [[0.5,0,0.5,1],[0.7,0,0.7,0.5],[0.8,0,0.8,0],[0.8,0,0.8,0]]
#startingpoint
@!stopsmoke
#dropsmoke
drop ["cl_basic","","Billboard",100,2,_relpos,[0,0,1],0,1.0,1.0,0.05,_size,_colours,[0],0,0,"","",_vehicle]
~0.05
?!stopsmoke: goto "dropsmoke"
goto "startingpoint"
exit