Customers can be deleted from the Customers page. Click on the Menu item "Customers". Then on the right side of the page click on the trash can next to the customer you'd like to delete.
To delete Customers in Bulk select all the Customers, with the box on the left side of the page, and then click on the "Delete" button at the top of the page.
To complete deletion you must type "Delete" into the dialog box to confirm deletion and then click on the "Delete" button.
After clicking on the "Delete" button the Customer will be deleted from your account.