The idea tickled my fancy, so I've put together a Chrome extension which detects if a page is using Google Analytics, and if so it injects a small script which sends bunk utm (urchin tracking module) data to the GA backend. It sets the traffic source to "DO NOT WANT!", and the campaign name to various things like "I do NOT want noisy ads!"

Here's the extension on the Chrome Store:

https://chrome.google.com/webstore/detail/do-not-want/pkejgbaojajbaimhneomoklkcejanpfd

For screenshots, more info, and if you want to have a good laugh at my shitty code, here's the repo:

https://github.com/sdmtr/donotwant

Note: before you jump all over me, yes, I realise this isn't going to make a huge impact on anyone and it's unlikely that whoever's monitoring the site's analytics will even notice the appearance of a new campaign with virtually no traffic.

It's just a bit of fun, although if you check the README in the repo you'll see a list of future ideas that I'd like to implement at some point which might actually make this extension useful to some degree.