I'd bet a month's wages that any spam you receive is either a) not sent from a valid address, making bouncing pointless, or b) sent from a valid address, the owner of which has no idea their address is being maliciously stamped in the from: field.
Hence these days, replying ain't going to make the slightest difference.
Spam exists because of the fundamentally flawed email architecture. It was designed a long time ago in much more trusting times.
Best thing you can do these days is invest £10/yr in a Spamcop subscription. I get around 300 bits of spam per day, but they all get filtered. About once a month I might see one which has slipped through the net and landed in my inbox, but I've never seen a false positive occur resulting in a legit email going into the spam box.