We've added a public API endpoint, POST /api/v1/flows/{flow_id}/conversations/{id}/complete, that marks a conversation as completed in a flow. The flow stops handling the conversation and any pending follow-ups are cancelled, so the person's next message goes to the inbox, just as when a flow runs out of steps. It works on active flows and on flows a staff member has taken over, and calling it on a conversation that's already completed changes nothing.