opencart_delete_customers.txt

While in the OpenCart back-end, navigate to Customers>Customers and then open up a console and type:
$('input[name*=\'selected\']').prop('checked', true);
$('#form-customer').submit();

This will remove all customers from the current page. 
(It's a quicker version of clicking to select all customers of the page and then the delete button and accept JS prompt box..)