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.
While in trial, the creator of the Chaser organisation has the ability to delete users, this is transferred to the subscriber once a paid plan is taken out.