Wouldn't it be much more effective to tell the advertisers why you are not trading with them any longer? If enough did this, I think the newspaper revenue would see a rapid drop.
If enough people cancel subscriptions it's just as effective. Newspaper advertising rates are set by the number of paid readers as verified by the Audit Bureau of Circulation (ABC). When advertisers realize their message isn't getting to a wide enough audience they tend to look for better ways to seek customers. Recent attempts by certain papers to inflate their circulation numbers are an indication of desperation.
Certainly letting advertisers know your feelings is important. But subscription cancellations really sting these days. I became fed up with the endless bias of our local paper recently and sent a note saying I wanted to cancel. Instead they kept leaving the unwanted paper in my driveway. Angry, I called and spoke with a really obnoxious woman who refused to acknowledge my demand. Instead she demanded to know why I'd want to cancel their wonderful newspaper. I gave her an earfull. Only when I told her I'd never make a another payment and if they didn't like it we could hack it out in small claims court, she finally sighed and said "Ok." That's how desperate they are to maintain paid circulation figures.
Sometimes it helps, sometimes it gets their backs up. What they can't ignore is declining profitability of their advertising dollars. As a bonus, the moonbats can't blame the losses on the "Religious Right."