Only allow checkout when customer in certain region/state

Hi All
I am starting to get to grips with Open Cart. Here is my store:

www.englandsfinestfood.com/store/index.php

We sell an English style frozen sausage product in the USA. As we can only deliver quickly to states in the US that are on the west side of the country, we would like to limit checkout to only those people who have registered and have an address in one of the states that we can ship to.

I know there is an option for creating a list of states GeoZones and Zones. I tried to set this up with just the states we can ship to, however I created a new test user with an address in a state that I didn't list in the Zones, and I was able to successfully checkout, which is not good.

Does anyone know the correct way to set this up so I can achieve the above?

Thanks in advance
Lawrence

remove the coutnries you don't want to ship to

Can I remove states I don't want to ship to also?

yes