Teacher modeling and enforcing the no put-down rule is critical. The rationale can be explained that every put-down usually wastes two minutes worth of time for a group because it destroys the group's effectiveness. The teacher can hold discussions on how people feel when they are put-down. Point out that most people's response to a put-down is to stop working and be angry or retaliate or evan sabotage the work. Short stories or historical examples of put-downs can be used to show how harmful they are.
Cartoons often give good examples of problems with put-downs.
After the class has gone a couple of days with no put-downs discuss the ways the classroom atmosphere and the quality of discussion has improved. It is good to alternate approaches. Hold a circle discussion. However, after about three contributions, randomly call on someone to tell what the first person said. Continue this procedure.