What stops countries from using the atomic bomb?

Answer 1

Nothing, you can't stop using something you haven't started using. The only country that has ever used atomic bombs is the US, that was to cause Japan to stop fighting during WW2; when they surrendered the US stopped using the atomic bombs on Japan.

If instead you really meant "What prevents countries from using the atomic bomb?"; that is a far more complex question to answer.

• Most countries are prevented from using atomic bombs simply because they don't have them. It costs a significant investment to set up the infrastructure needed to make them and few countries wish to make such an expenditure when they have higher priorities.
• Formally, many countries are prevented from using atomic bombs because they have signed treaties against using them. But they could either choose to withdraw from or abrogate these treaties at any time, so such treaties only prevent a country from doing what it isn't planning on doing anyway.
• Some countries are prevented from using atomic bombs by public opinion.
• It has frequently been claimed that fear of nuclear retaliation prevents countries armed with atomic bombs from using them on each other. One form of this idea is called MAD. However one can never be certain that a leader with no such fear might come to power in a country and just attack anyway.
• etc.