Returning Junkmail

I wonder if it's legal to return junkmail. I don't mean all of it, that'd be expensive. I mean the crap that my phone company puts in the envelope with my bill...if I put it right back in the reply envelope with my check, could they claim some sort of harassment? Admittedly, my goal is to make it expensive enough for them that they stop sending me junkmail I can't avoid. Seems like turnabout should be fairplay, but I know there've been several legal cases where customer-pranks against billing departments were found to be illegal.

Still, seems like I should have some sort of recourse. Maybe if I mark each piece I return with a request for them to remove me from the extraneous mailing, it'll count as legitimate correspondance.

All I really want is to stop getting the junk.