Of course, but we already do this. I block all ads, which means I'm modifying your app to not make any money off of me. Such is the reality of the Internet and general purpose computers.
No, you just think you block ads. It's extremely naive to think you actually block all ads. I'm sure a lot of people have made money out of you online from advertising you don't recognize as advertising.
edit: Sure, downvote me if you like. If you like to think adblock etc actually block all adverts I guess it's up to you. It blocks a small class of 'obvious adverts'. Several websites will pass outgoing links through a jump script which may or may not then go through affiliate links. Adblock is useless against such things. There are several such examples.
That's fine. I use flashblock and adblock to block annoying flashing animated noisy colourful page disrupting ads, to block ads and ad servers tracking me and giving them power and commercial gain with no gain to me, and to stop FireFox playing noise from background tabs I can't find quickly.
I don't do it to stop site owners 'making' money from me. If you can get non-annoying ads to me, and make money from me, that's brilliant. If that's all anyone did, I'd get rid of *block altogether.
If you can arbitrarily modify the server side of any instance of the application then you can also just pay using someone elses credit card details.
Although I think what Stallman would probably prefer is the source to the site / web application being available so that people can run their own instances.
HN and reddit have their source available. I don't see anyone using the reddit code in a way that's any threat to reddit, mainly since the size of the community is an asset that's hard to compete with.
Potentially off-topic, since I'm not familiar with the prevalent licensing, but the various django applications look pretty nice for this kind of thing.