Just remind you it's a module 'base_import' you should install

as generally written here http://help.openerp.com/question/19174/delete-all-customers-at-once/ it may not be recommended if you already used them in sales, purchases, invoices etc. If not you can just mark and delete them all this way (Ver 7.0): Open tree view with data - maybe filter data - make the treelist show you all data at once (klick on the e.g "1 - 80 of 8000" upper right corner - mark them all by checking the very top checkbox in the very left column - use delete from "more" dropdown menu That's it.