Used to actually do this back in the day. I would send them back their offer with something like "I regret to inform you that I am not interested in your offer. Have a nice day." on the top page. I figured that going out of my way to let them know that they shouldn't expect a sale from me was simply being polite. I used their handy return envelope to send it back to them.
There were stories of people who would attach their return envelopes to heavy objects and try to send those back to them.
Maybe some day advertisers will come to understand that if their ads affect me negatively, then I will do my best to make it unprofitable for them to present them to me.