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.
It was both: an atomic bomb using uranium as its fuel.
The atomic bomb is too expensive to develop for a city to have them. They are weapons held only by industrialized countries.
many countries
The A-Bomb was invented in 1945.
four million people would have did if we did not use the atomic bomb
They were dropped using airplanes.
Russia (USSR) declared war on Japan right after the Hiroshima bomb.
Many countries use atomic energy to generate electricity
Hiroshima and Nagasaki
Spies
Wether it was ethical or not to use the atomic bomb on other countries because of the destruction they cause.
If you means BESIDES America- in 1945, no one.