To delete an existing flow, click on the Flows tab and find the flow you'd like to delete. In the dropdown next to the Edit Details button, click Delete.
When a flow is deleted, no future recipients are queued and all queued recipients are cancelled. You will also delete the analytics associated with the deleted flow. If you don't want to delete a flow, but don't want to send future emails, you can pause flow emails instead by switching the flow to draft mode.