This feature is available on Premium and Growth plans.
Set a cutoff time for volunteers cancelling via the hub. (ie. 24 hours before their event or shift). While this won't stop people from cancelling altogether - this feature will give you more control and the ability to communicate your preferred process for cancelling last minute.
Simply go to your Organization Settings in the bottom left corner, click Preferences, and indicate the number of hours they have to cancel. You can also include a custom message with details.