If Russia or China decided to open the bank and supply country with nuclear technology and deliver systems there is nothing we can do. North Korea is a case in point. I would bet their entire program is based on China's giving them technology. Iran is another interesting situation.
You made a good point. I realize there is a lot of behind the scene support for preventing Iraq and Iran from getting nukes.
The technology is known. It is a matter of engineering and economics.
Nothing can be done to stop South Africa, Indonesia, Vietnam, Thailand, Brazil, Argentina, Mexico, Egypt, Nigeria of the world are too large to be prevented by any kind of military coercion. You are really relying upon moral persuasion. Fortunately those countries have a lot of sane people.