It cannot be stopped or prevented. The only thing that can done, as Wayne LaPierre pointed out, is to have a good guy with a gun there to kill the bad guy.
I agree that evil behind this massacre and others will always be with us. But the truth is, good guys with with guns are mostly needed when a society allows evil to take root and grow.
There are a number of good lessons presented during this broadcast that makes it worth the time to view.