The defender received a second yellow card during his team's 2-1 win over Brechin on Saturday after kicking the ball against an opponent after play had stopped.
The former Southend player also got a second yellow card after the January 2 game against Airdrie following a clash with Diamonds boss Gary Bollan.
Mohsni will now miss League One games against Ayr and Stenhousemuir this month through suspension, and he is just one yellow card away from another ban.
McCoist told the club's official website: "I was very angry and disappointed on Saturday. Bilel knew that and the sending-off was silly and needless. It put us under a little bit of pressure.
"We had a good chat about it this morning and he's in total agreement with the sending-off, as you would expect. It was a silly thing to do but now we move on.
"We don't want it to happen again obviously and hopefully we can learn lessons from that particular incident.
"Bilel is like Madjid Bougherra was in many ways. They're both really passionate about their football and they react differently when they think an injustice has taken place.
"There was certainly no injustice in the sending-off, though. We just have to direct Bilel's passion and his natural aggression and sometimes he needs to take a step back.
"If that means counting to 10, he needs to count to 10. As I've said to him today, he's no use to us if he's suspended."