Finding proximity by latitude and longitude

I am making a market type application where users enter product information and product address, now I geocode that address into latitude and longitude using Google Geodcoder and then process that information in a MySql database. Proximity search works flawlessly as long as I give the user a specific radius to search in the immediate vicinity. However, I cannot determine how I will handle it at the border of the country. I do not want people from India to be able to see the product from Pakistan, it will be a disaster for my statement as both are at war. Any suggestions as to how I can overcome this.

I had other suggestions too : https://dba.stackexchange.com/questions/39115/zip-based-search-in-mysql/39138?noredirect=1#comment69710_39138

However, I want the best and most efficient way to do this. Any help would be much appreciated.