There is a pretty clear line here surely?
It is fine to just send it back as that inconveniences the company and may even lead to them having to pay more out to staff in either overtime or temp costs, or perhaps if they are too tight to do that, something else won't get done.
Anything that means that the 'little people' who actually have to process the mail have to deal with hazardous or just downright disgusting stuff isn't on - this kind of thing hits so wide of the mark it is untrue.
The real issue is how to register your disapproval in such a way as it inconveniences the cynical shysters at the top. This is much more difficult as they will have spent a lot of time insulating themselves from this kind of thing. I bet no one has even bothered to mention this to the 'top floor'.
The only legal way you can 'hurt' the people at the top is to hit them in the pocket - but this is very difficult. I guess for the likes of the Sun the easiest way 'in' is to expose some kind of malpractice or opinion that puts them at odds with the ethos of companies that pay for advertising(?)
Almost all knee-jerk responses hit those who are directly in the firing line - and that is very rarely the 'top floor'.
This is why Leveson was/is a good thing - finally someone dragged these scumbags out of their ivory towers, put them in front of some cameras for a change and made them squirm.