precedent? This is already a common practice. Since early 2010s ISPs were doing it in USA [1] and trying to lower net neutrality standards in Europe to do it [2]. That's how the whole net neutrality debate started in US https://en.wikipedia.org/wiki/Ajit_Pai#Net_neutrality_in_the...
OP was suggesting an outright block of traffic from ad providers. I don't believe any US ISPs have gone to that extreme, at least not yet. They allowed peering points to become congested which affected Netflix traffic, but weren't blocking it.
[1] https://www.nexttv.com/news/netflix-ceo-some-big-isps-extrac...
[2] https://epicenter.works/en/content/data-toll-deutsche-teleko...