The ISP could drop or throttle the traffic, assuming that's allowable under the ToS with their customers. But then of course their customers will quit and find an ISP that doesn't throttle/drop.

Regardless, Netflix is not pushing content. The consumer is pulling it.

As a one-off, issues like the Squid-Game aren't so bad, but if it's a chronic problem for the ISP then it's because their model of consumer consumption was wrong.