It's something that Gmail & Calendar do. Nothing to do from our side, it depends on every user to disable the option in their inboxes and calendars.
Basically with numbers! Numbers are always the best way to show why it is important to get a tool, project, process. Showing the current numbers/situation, what its wanted to achieve and how much time/money its necesary to achieve that goals. If numbers are solid, there is your next email marketing level! :D