The point is not to stop your ISP from showing you ads. The point is to stop your ISP from interfering with your traffic in transit.
If I ran a website with ads, and someone was stripping those ads to replace with their own ads, I'd be annoyed. I'd be amazed if that's something Google would tolerate. We've seen plenty of stories from people saying "Google closed my ad account and froze all my money!!!" so I hope they do that to this ISP and or the company serving the ads.