Table rate shipping defaults to 0.00 when final amt is exceeded

I have the shipping module set up and working properly using three table rates for three zone groups. I have one problem though:
when the customer purchases more than my final # of products defined for shipping (the shipping is based on # of products purchased), the shipping comes up as "The shipping rate cannot be determined at this time" and a "$0.00" shipping charge. And if the costumer continues the checkout, it allows him to proceed and confirm/place the order with a 0 shipping charge, which my client doesn't want. We want it to not allow the processing of an order where the number of items in the cart exceeds the final amount defined in the shipping table.
Any thoughts?

If you're still confused - say my shipping table for the US (by quantity of items purchased) is this:
5:5, 10:10, 15:15

And my US customer puts 17 items in his cart and tries to checkout - he gets the "shipping rate cannot be determined" in his checkout screen with a $0 cost for shipping and is allowed to check out this way. I want it to prevent him from being able to place the order with an error of "shipping rate cannot be determined - please email us regarding your order first" or something like that.

Thanks so much for any help.
Ellie

You would need some validation code added to this but I am mystified why the client wants to turn away business??
Why not allow 20-30-40 items to be purchased if that is what the customer wants