Ilker,
I am not sure if there's a way to cancel the add to cart button for any product. I had looked at that page before and I know the add to cart is a submit button for the form it's on. You would have to change it somehow to a button and not a submit and have it run a function that checks quantity. Sorry, I don't have details on how to do that.
Have you tried posting to the Virtuemart forum about this?
www.virtuemart.net.
Regards,