In general: American football players often throw the ball quite vigorously to the ground after scoring a touchdown. this is known as 'spiking'.
Specifically: When President Obama used the term, he was talking about his decision not to show the photographs of a dead Osama bin Laden, which would be tantamount to spiking the football.
In this context, "Spiking the football" meant that it would have been a brutal, useless and callous act to have published photos of a dead and mutilated corpse.
When a football player spikes the ball in the endzone he is also giulty of "overkill". To score, the player only needs to get the ball into the inzone and have it touch the ground. Some players, however, like to show off and slam
In the US, when a football player makes an absolutely awesome touchdown, he often spikes the ball in the endzone and strikes a victorious pose. Team fams go wild!
Some say that it is an excessive show, but they would be fans of the losing team!