I think the community is doing a great job by rapidly closing off-topic question, thus preventing answers, and it's enough.
Additionally I would down-vote the question (with appropriate comment when possible) and not the answer.
I agree with Teun Vink, that this could discourage people which are potentially great members and that's worse than having a few answers to off-topic question.
On a side note, there's some question that are off-topic - mainly because they are educational questions- but well asked, giving all relevant details and showing some research, and those are generally better received. Should we restrain ourselves from answering those also?
I feel that in this specific case we can answer while at the same time educate the user about what is on-topic and how to use this (great) site.