It is very difficult to stop the chain reaction of an atomic bomb since the spontaneous fissions always occurring within the material initiates the chain reaction. However, there are a number of bomb experts who can easily stop the detonation of a bomb before an explosion takes place.
It can't be stopped! That's why it is called a CHAIN-reaction. Its not like dominoes where you can pull a piece and stop the reaction. It's. at the atomic level and the reaction occurs at nearly the speed of light.