problem with google indexing

I recently coded an E-commerce site from scratch (no use of drupal or any other CM generator and no use of these shop fucntions that some ISPs provide)

The CM system which i gave the client allows him to add both brands and product types.
Obviously it also lets him add edit and delete individual products.

The "add product" system provides a drop down menu for both brand name and product type
the options for it are sourced from the brands a product types which he has added to the system.

The front end of the site which the visitors see has several menus such as the main HTML menu, various drop down menus ect, all of which source the options they display from the list of brands and product types which the client has made.

product types are always a sub menu which appear bellow each brand

One important aspect of the system is that for all the menus if a particular combination of brand and product type would return no products the site does not display that option to the user.

so in the main menu no brand has a sublisting for a product for which it currently has 0 list

ANYWAY MY PROBLEM IS THIS
the hyperlinks send variables to the product listing page using GET, which means the data is part of the URL
and since they are URLs google can index them.
and said indexed searches can remain out of date for several months

this means a google search could still result in the display of a page which show no prducts

how can i prevent this?