Allow multiple countries for billing but restrict for shipping

Question

I have a site and I would like to allow people from three different countries to be able to shop, but want to (for now) restrict shipping to just one country (so someone in a foreign country can buy from the store and have it sent to a friend or family member in the local country). I read here about changing the specific countries in the shipping method. I tried it, but I'm still getting all three allowed countries in both the billing and shipping address.

I'm not sure whether the "ship to same address" checkbox has something to do with it, since it's checked by default. Say my allowed countries are Spain, USA and Guatemala, but I want to have shipping available only in Guatemala. If I select USA for my billing address the checkbox should uncheck itself because I can't use the same address for shipping. Also, perhaps it has something to do with the fact that to this point I haven't selected a shipping method yet, so the shipping address country dropdown displays all three countries.

What am I doing wrong? Is what I'm trying to do possible from the admin?

Note: I'm using the OnePageCheckout extension provided by Apptha. Also, I'm new to this forum.