There is a Chinese proverb for this (of course):
Sha Ji, Jing Hou. "Kill a chicken to scare the monkeys".
Another tactic that I use that when doing groupwork is require each group to first choose a captain (or chicken). Then I tell them that the captain's job is [here insert whatever leadership duties are necessary for the task]. Finally I tell them that the captain's MOST important duty is to make sure everyone in the group speaks only English while preparing the task. If not, I tell them, I will "kill" the captain. Then I point to each captain and ask them if they want to be the chicken that is used to scare the monkeys.
You'll find virtually zero Chinese being spoken thereafter. The captains (cum chickens) will take their role seriously. (This all might sound harsh in writing but it's all done good-naturedly. And your students will be impressed that you know "Sha Ji, Jing Hou".)