Those quick mysql scripts, will help you to remove all data from a Magento installation.

• Customers and its related data (including logs):
  
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  use <database_name>;   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;

• Sales orders
  
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  use <database_name>;   SET FOREIGN_KEY_CHECKS=0;   TRUNCATE `sales_flat_creditmemo`; TRUNCATE `sales_flat_creditmemo_comment`; TRUNCATE `sales_flat_creditmemo_grid`; TRUNCATE `sales_flat_creditmemo_item`; TRUNCATE `sales_flat_invoice`; TRUNCATE `sales_flat_invoice_comment`; TRUNCATE `sales_flat_invoice_grid`; TRUNCATE `sales_flat_invoice_item`; TRUNCATE `sales_flat_order`; TRUNCATE `sales_flat_order_address`; TRUNCATE `sales_flat_order_grid`; TRUNCATE `sales_flat_order_item`; TRUNCATE `sales_flat_order_payment`; TRUNCATE `sales_flat_order_status_history`; TRUNCATE `sales_flat_quote`; TRUNCATE `sales_flat_quote_address`; TRUNCATE `sales_flat_quote_address_item`; TRUNCATE `sales_flat_quote_item`; TRUNCATE `sales_flat_quote_item_option`; TRUNCATE `sales_flat_quote_payment`; TRUNCATE `sales_flat_quote_shipping_rate`; TRUNCATE `sales_flat_shipment`; TRUNCATE `sales_flat_shipment_comment`; TRUNCATE `sales_flat_shipment_grid`; TRUNCATE `sales_flat_shipment_item`; TRUNCATE `sales_flat_shipment_track`; TRUNCATE `sales_invoiced_aggregated`; TRUNCATE `sales_invoiced_aggregated_order`; TRUNCATE `sales_payment_transaction`; TRUNCATE `sales_order_aggregated_created`; TRUNCATE `sales_order_tax`; TRUNCATE `sales_order_tax_item`; TRUNCATE `sendfriend_log`; TRUNCATE `tag`; TRUNCATE `tag_relation`; TRUNCATE `tag_summary`; TRUNCATE `wishlist`; TRUNCATE `log_quote`; TRUNCATE `report_event`;   ALTER TABLE `sales_flat_creditmemo` AUTO_INCREMENT=1; ALTER TABLE `sales_flat_creditmemo_comment` AUTO_INCREMENT=1; ALTER TABLE `sales_flat_creditmemo_grid` AUTO_INCREMENT=1; ALTER TABLE `sales_flat_creditmemo_item` AUTO_INCREMENT=1; ALTER TABLE `sales_flat_invoice` AUTO_INCREMENT=1; ALTER TABLE `sales_flat_invoice_comment` AUTO_INCREMENT=1; ALTER TABLE `sales_flat_invoice_grid` AUTO_INCREMENT=1; ALTER TABLE `sales_flat_invoice_item` AUTO_INCREMENT=1; ALTER TABLE `sales_flat_order` AUTO_INCREMENT=1; ALTER TABLE `sales_flat_order_address` AUTO_INCREMENT=1; ALTER TABLE `sales_flat_order_grid` AUTO_INCREMENT=1; ALTER TABLE `sales_flat_order_item` AUTO_INCREMENT=1; ALTER TABLE `sales_flat_order_payment` AUTO_INCREMENT=1; ALTER TABLE `sales_flat_order_status_history` AUTO_INCREMENT=1; ALTER TABLE `sales_flat_quote` AUTO_INCREMENT=1; ALTER TABLE `sales_flat_quote_address` AUTO_INCREMENT=1; ALTER TABLE `sales_flat_quote_address_item` AUTO_INCREMENT=1; ALTER TABLE `sales_flat_quote_item` AUTO_INCREMENT=1; ALTER TABLE `sales_flat_quote_item_option` AUTO_INCREMENT=1; ALTER TABLE `sales_flat_quote_payment` AUTO_INCREMENT=1; ALTER TABLE `sales_flat_quote_shipping_rate` AUTO_INCREMENT=1; ALTER TABLE `sales_flat_shipment` AUTO_INCREMENT=1; ALTER TABLE `sales_flat_shipment_comment` AUTO_INCREMENT=1; ALTER TABLE `sales_flat_shipment_grid` AUTO_INCREMENT=1; ALTER TABLE `sales_flat_shipment_item` AUTO_INCREMENT=1; ALTER TABLE `sales_flat_shipment_track` AUTO_INCREMENT=1; ALTER TABLE `sales_invoiced_aggregated` AUTO_INCREMENT=1; ALTER TABLE `sales_invoiced_aggregated_order` AUTO_INCREMENT=1; ALTER TABLE `sales_payment_transaction` AUTO_INCREMENT=1; ALTER TABLE `sales_order_aggregated_created` AUTO_INCREMENT=1; ALTER TABLE `sales_order_tax` AUTO_INCREMENT=1; ALTER TABLE `sales_order_tax_item` AUTO_INCREMENT=1; ALTER TABLE `sendfriend_log` AUTO_INCREMENT=1; ALTER TABLE `tag` AUTO_INCREMENT=1; ALTER TABLE `tag_relation` AUTO_INCREMENT=1; ALTER TABLE `tag_summary` AUTO_INCREMENT=1; ALTER TABLE `wishlist` AUTO_INCREMENT=1; ALTER TABLE `log_quote` AUTO_INCREMENT=1; ALTER TABLE `report_event` AUTO_INCREMENT=1;   SET FOREIGN_KEY_CHECKS=1;

• Categories
  
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  use <database_name>;   TRUNCATE TABLE `catalog_category_entity`; TRUNCATE TABLE `catalog_category_entity_datetime`; TRUNCATE TABLE `catalog_category_entity_decimal`; TRUNCATE TABLE `catalog_category_entity_int`; TRUNCATE TABLE `catalog_category_entity_text`; TRUNCATE TABLE `catalog_category_entity_varchar`; TRUNCATE TABLE `catalog_category_product`; TRUNCATE TABLE `catalog_category_product_index`;   INSERT  INTO `catalog_category_entity`(`entity_id`,`entity_type_id`,`attribute_set_id`,`parent_id`,`created_at`,`updated_at`,`path`,`POSITION`,`level`,`children_count`) VALUES (1,3,0,0,'0000-00-00 00:00:00','2009-02-20 00:25:34','1',1,0,1),(2,3,3,0,'2009-02-20 00:25:34','2009-02-20 00:25:34','1/2',1,1,0); INSERT  INTO `catalog_category_entity_int`(`value_id`,`entity_type_id`,`attribute_id`,`store_id`,`entity_id`,`value`) VALUES (1,3,32,0,2,1),(2,3,32,1,2,1); INSERT  INTO `catalog_category_entity_varchar`(`value_id`,`entity_type_id`,`attribute_id`,`store_id`,`entity_id`,`value`) VALUES (1,3,31,0,1,'Root Catalog'),(2,3,33,0,1,'root-catalog'),(3,3,31,0,2,'Default Category'),(4,3,39,0,2,'PRODUCTS'),(5,3,33,0,2,'default-category');