It's a chain. You first contact the leaker and their upstream, and then if that doesn't work then their upstream, etc.

At some point you reach a company that's large enough that they must cooperate because they want to remain in business of being an actual responsible ISP.

And then there's Verizon, who can safely ignore any ISP etiquette because they have a de-facto monopoly.

It's sort of a network of trust thing, every time this happens everyone has to scramble to add route filters to ignore the leaked route on all their routers, and then they try to contact the leaker in parallel get them to fix it as well (and their upstream routers).

https://www.noction.com/blog/bgp-hijacking

The upstream provider, if cooperative, could filter out their announcement as a quick fix. It's surprising it happened though, most upstreams put filters in place already.