1) Remove statistics information from dashboard.
Click NO to ‘Demo Mode’ on your Dashboard. It’s on the lower right hand side.
2) Remove all sample and demo orders, catalog etc from the database
Go to the “Modules > Modules” page.
Find the “Database Cleaner” module and click its “Install” button.
You are directly taken to its configuration page (if not, click its “Configuration” button).
Read and accept the warning, then click the “Delete Catalog” button: it will delete all your products and their attributes, manufacturers, etc.
Read and accept the warning, then click the “Delete Orders & Customers” button: it will delete all your customers and their orders, carts, etc.
Click the “Check & fix” button to refine your database integrity constraints.
Click the “Clean & optimize” button to reorganizes the physical storage of table data and associated index data, to reduce storage space and improve I/O efficiency when accessing the tables.
THERE IS NO WAY BACK. Be sure to only click these buttons if you do intend to wipe your database from its default content.
Source: http://www.prestashop.com/forums/topic/320579-delete-all-sample-data/