The first thing is, how do you distinguish between the ad and a non-ad? If my friend mentions a famous brand in a chat with me, should there be some entity to remove the name of that brand from the chat, so that I don't see it?

There are many videos on Youtube recommending stuff. You never know if the author has been paid. Even if you try to detect popular ad networks and services, the ad techniques will also evolve.

If ads were the only source of my income, and my content was unique, I would show the user a "quiz" every 5 minutes, asking him to answer, what is shown in the ads at the moment :D and deny the access, if the answer is wrong.