youngump wroteI don't think the ball is dead for defensive USC. As long as things are settled runner isn't in need of immediate medical care, wait for the play to end. If you do end it early, then I think the right thing to do would be to place runners where they would have reached if not for the USC similar to how an obstruction dead ball is ruled.
As far as I know the rule book is silent on this. But somebody will be along to correct me soon enough if it isn't.
The only times you should ever kill the play are if A) a player needs immediate medical attention or B) if the play has clearly ended and softball is no longer being played (as in a bench-clearing brawl), and thus, the play is over. Otherwise, you have no justification for killing the play.