I agree with Vid- reduce/withhold subsidies or tax breaks for any group that enforces a shunning policy.
As for 'shunning is a crime,' I don't think it's possible to make it a crime on an individual level. Perhaps you could make it a crime for it to be an official/documented organizational policy, and require organizations or businesses to remove/amend such policies or rules. But individuals shun one another all the time, for any number of reasons. It would be impossible to legislate.