Limit location sign ups

Dear Sirs,

I only want to send my items to customer based in countries that only have interest to me i.e UK (Where I live), USA, OZ.

If someone from Thailand attempts to join my store they won't be able as I do not want to send my products to that country.

How do I do this?

Would be easier just to restrict shipping to Geo Zone and deny zones that are not in UK. This way you could allow people to sign up but not checkout. If you want people to not register based on their location then you are talking about a completely different thing and there is no extension for it. You would need to get help to implement the validation