If you mean real spam, like those trying to sell you pills or scamming / phishing information out of you, bouncing the email won't have any effect at all. The messages are usually sent from botnets (ie: networks of hundreds of infected computers controlled by the spammer), the reply addresses are faked, etc...
If, on the other hand, you mean regular websites that have acquired a database of harvested email adresses and are spamming you with their newsletter, then either they care for the quality of the base and they'll usually provide you with an unsubscribe link, or they don't and there's no point in bouncing back the mail. It will just be ignored. Best thing to do against them still is legal action, since they usually cannot / will not hide their real identity, like professional spammers do.
Conclusion: automatic spam classification is still the way to go, and I must say that gmail is good enough at it that I'm rarely bothered by spam in my inbox.