Or perhaps I'm getting a false positive and both of the above posts were serious.
Personally, I like that every time I move, my junk mail drops off for many months (the "occupant" garbage still shows up, but it's easier to toss without thinking about it). I think that's the only thing I like about moving, actually. Having a system that allowed junkmailers to know my new address automatically would be a negative value, and I think the same is true of a site that allows constant contact with folks on vacation--I like being out of reach when on vacation.
I'm afraid I'm just not willing to invest that much time in costing them money--because the amount of money it costs the sender is significantly lower than the value I put on my time. Sure, if enough people do it, they will eventually find it unprofitable to send their junk...but the human resources wasted in getting to that point is overwhelming. Maybe I should hire a neighborhood kid to come by weekly, for envelope stuffing. $5 per week might be worth it.