Magento 1 - Remove all Customers in Magento

Make a BACKUP and test on a dev server first! This will DELETE all customer data including logs.

SET FOREIGN_KEY_CHECKS=0;
-- reset customers
TRUNCATE customer_address_entity;
TRUNCATE customer_address_entity_datetime;
TRUNCATE customer_address_entity_decimal;
TRUNCATE customer_address_entity_int;
TRUNCATE customer_address_entity_text;
TRUNCATE customer_address_entity_varchar;
TRUNCATE customer_entity;
TRUNCATE customer_entity_datetime;
TRUNCATE customer_entity_decimal;
TRUNCATE customer_entity_int;
TRUNCATE customer_entity_text;
TRUNCATE customer_entity_varchar;
TRUNCATE log_customer;
TRUNCATE log_visitor;
TRUNCATE log_visitor_info;

ALTER TABLE customer_address_entity AUTO_INCREMENT=1;
ALTER TABLE customer_address_entity_datetime AUTO_INCREMENT=1;
ALTER TABLE customer_address_entity_decimal AUTO_INCREMENT=1;
ALTER TABLE customer_address_entity_int AUTO_INCREMENT=1;
ALTER TABLE customer_address_entity_text AUTO_INCREMENT=1;
ALTER TABLE customer_address_entity_varchar AUTO_INCREMENT=1;
ALTER TABLE customer_entity AUTO_INCREMENT=1;
ALTER TABLE customer_entity_datetime AUTO_INCREMENT=1;
ALTER TABLE customer_entity_decimal AUTO_INCREMENT=1;
ALTER TABLE customer_entity_int AUTO_INCREMENT=1;
ALTER TABLE customer_entity_text AUTO_INCREMENT=1;
ALTER TABLE customer_entity_varchar AUTO_INCREMENT=1;
ALTER TABLE log_customer AUTO_INCREMENT=1;
ALTER TABLE log_visitor AUTO_INCREMENT=1;
ALTER TABLE log_visitor_info AUTO_INCREMENT=1;
SET FOREIGN_KEY_CHECKS=1;

Mage::register('isSecureArea', true);

$customers = Mage::getModel("customer/customer")->getCollection();

foreach ($customers as $customer) {
    $customer->delete();
}

Be sure to know what you are doing though.. You can disable the customer as well if delete is not what you need.