Once subscribed to a paid plan, the subscriber has the ability to delete other users. You can delete users by going to Settings > Users, and then clicking the 'Delete' button next to the desired user. This will remove the user from the organisation. Deleting a user will also remove them as a sender of the selected organisation.
Users that are currently assigned as a sender (normal, escalated or cc'd) cannot be deleted. Users that are creators or subscribers of the selected organisation cannot be deleted.