I've tried this before, writing "return to sender" on spam, except was putting them back in the mailbox with the flag up. The mailman threw a minor fit after the 3rd or 4th time I did it, left a note in the mailbox complaining about it, and started circling with a pen "or Current Resident" on every piece of junk mail. I quit after that because pissing off the mailman is not worth it and I wasn't about to take the fight anywhere past that.