Accidental early detonation
As stated in other answers stopping the arms race from a technological point of view is hard to impossible. Making scientist and/or the general public unwilling to fund/participate in the development is probably easier. For nuclear power this has precedent in the Chernobyl accident which has had a severe impact on all research into nuclear power that still persist today.
The accidental early detonation of the (would be) first atomic bomb while still at the production facility would cause a major set back in the research since it both destroys evidence of what has gone wrong and kills a lot of the scientist and engineers necessary to build the bomb in the first place. Since it will be hard to find out what has gone wrong it will be hard for the scientist to say if it was a minor problem or a major design flaw. The effect will be that anybody will be very hesitant to continue with the project or try to start a new project.
For the disclaimers:
* An accidental atomic explosion can be caused by a simple very small effect such as a faulty wire presumably. 435
Although I am not sure what fail safe mechanisms they had in place during production of the atomic bomb.
* This scheme would set back the USA but not necessarily the USSR although scientist there might become quite nervous about development. On the other hand if the USA didn't develop the atomic bomb the USSR might be less inclined since there would be no mutually assured destruction arms race.
* Delaying it to 2010 might not be possible with a single explosion but should come from later treaties. The UN banning the development of nuclear weapons due to the risky nature of it linked to the explosion seems feasible.
* You could argue that an atomic explosion is in fact an atomic bomb, but I would say that for it to be a "bomb" it should be stable and controllable which was not the case in this scenario, thus it should not qualify as an atomic bomb.
* As a minor side note, but that would completely depend on the story. With no atomic weapons race between the USA and the USSR they have ample opportunities to get into conflict anywhere else, be it normal war, space or anything else. The space race could for instance give such a drain on both of their resources that they wouldn't have enough resources left for the development of an atomic bomb making the year 2010 more viable. 477