Simon: Think it through, how would it work?
Simple. Make it illegal for any organization to incite - and especially coerce - its members to shun former members. If members of such an organization are punished or in any way treated differently or negatively by the organization, for failing to shun former members, then those responsible are subject to prosecution.
Individuals still have the freedom to shun if they want. But they can't be instructed to do so by the org and they can't be punished by the org if they don't.