Do you want to remove quantity availability from just these pre-order products or all products?
In the end it would require a sql statement to modify, but you could setup the presale product as product type document-general where the quantities available are hidden for document-general product type, then when the product does come into availability it can be changed over to a product-general product type using a sql command in tools->install sql patches.
Or you could code to exclude display of the product quantity availability for particular products or products in particular categories. Or you could use css to visually hide the available quantity (html through view source would still have the available quantity present.)
The sql command to change the product type of the product having products_id=5 back to product-general is:
Code:
UPDATE products set products_type = 1 WHERE products_id = 5;