At the risk of being "flamed," here's my personal opinion on how to handle JW trash (literature) that is found in public places:
I think it is inappropriate to simply take or dispose of the magazines. In my mind, that is the equivalent of book-burning. Who appointed us the judge of what people should be able to read? How is it we get to decide what the public does or does not have access to? We may think we are "saving" the public from a destructive, mind-controlling cult (and we ARE), but it is not our place to take or destroy things that do not belong to us. Yes, they were left out for the public to have at no charge, but they were not intended to be collected wholesale and disposed of because WE don't like the message.
That said, the magazines themselves (if read by anyone with a modicum of reasoning ability) do a fine job of anti-witnessing on their own. Yes, they are full of self-aggrandizing propaganda, dumbed down "science," and inane religious doctrines that require mental gymnastics just to follow their line of "reasoning." And that's my point: anyone with just a little education or common sense will likely see through them for the pointless drivel they are.
But here's what I DO think would be fair and appropriate: do as others have done and put stickers on them pointing to websites that show just how silly and false WT teachings are. That way we are not interfering with others' rights to read what they wish, and we're not taking things not meant for us for our own purposes. We're just showing the public that there is another point of view while warning them of the dangers of Watchtower Society. I doubt very many would even talk to the Witnesses again if they visited a site like this just once.
Collecting and destroying literature, just like losing our temper and screaming and yelling at JWs, may do us and our "cause" (warning people about the Watchtower) more harm than good. One way to make sure people read something is to try to supress it. If someone picks a JW rag to read and notices an "apostate" sticker and then checks it out, our work is done. Be sure to count your time!