The former Hearts and Inverness midfielder was dismissed for a second bookable offence towards the end of the 1-0 victory which lifted Rangers to within three points of the third division title.
Now Johnstone is looking for an improvement in the Scotland international's onfield behaviour after he took his card count for the season to 12 yellows and two reds at Borough Briggs.
"Ian could have seen red long before he did on Saturday after a number of fouls," he told the Evening Times.
"The one he did get his second booking for was maybe a bit harsh but, at the end of the day, he has got to calm himself down and be smarter.
"You are never going to change a referee's mind so I don't know why he keeps on at them throughout games if he doesn't get a decision.
"Most of his bookings this year are for silly things like kicking the ball away or bouncing it in anger when he doesn't get a decision.
"That kind of thing is just stupidity and that will cost you in the long run when you get banned.
"You don't mind players who are passionate about the game, but that passion has to be about your own team and doing what is right, not taking it out on referees or other players and getting in trouble.
"There is only going to be one winner when you start doing that - and it isn't you."