To reset the availability of products, first of all recalculate inventory availability, menu-> edit-> calculate availability/sales.
Then open purchases/sales and choose a document that exports goods such as a delivery note and create a new document.
Select your company as a customer and then, to massively add all products, in the first row of items, double-click and open the inventory/products/services.
Press ctrl + a to select all products (or select only those that you want to reset by pressing ctrl) and close the window by clicking on the top right.
In the question “Enter in the quantity field the total availability of the item” choose yes and wait until the products be imported into the new document.
Finally, save the document and finally quantities are zero!