Speak
And thats a very valid point, one we have not discussed yet. A cat call from typical scenario ie building site, night club, on the street with groups of young men, or as the OP stated a group of young boys etc is childish and need adressing.
A guy on his own is a different story, and his actions can range from perfectly innocent and attempting to start a conversation, to very sinister. I would be more wary of this type, however would a man run for his life if a woman said something to him on the street? I am not scared of men in general, and would comfortably confront him, given the scenario obviously.
I'm not sure who mocked me for trying to understand why people do what they do, but it's quite simple. Criminal investigators study criminal psychology, in order to understand why someone does something you can recognise how to stop them doing something, the OP asked how do we stop this?
Like every other problem, we understand why it happens and work out how to stop it from happening, if you think having the victim decide the justice then the problem will always persist.
The other issue is that some women actually like it to a point, react positively, and encourage it. So if we are to acknowledge everyones right to think and feel what they want, then who is right, the opposer of cat calling, or the promoter?