Validate quantity by individual product

Hi,

How can I validate quantity for specific products or set max value. I have set for some products to allow only one purchase by order, so it works in well on the cart page but not before, selecting the quantity. It should not let me increase the quantity product in more than one, and neither add it to the cart.

Thanks , I hope you can help me.

The page I need help with: [log in to see the link]