Easy for the gov to do it: make hosting a gambling website in the US illegal, make providing access to online gambling illegal, treat money earned from gambling services like money earned from narcotics trafficking, threaten to punish payment processors if they work with gambling providers, make gambling commercials illegal (like they did with tobacco) etc. Make it where you can't run a billion-dollar company like DraftKings in the US and watch it dry up overnight.

Online gambling was literally illegal in most of the US a decade ago, even on the modern post-social-media internet. You could still do it by jumping through hoops like using a VPN to access foreign gambling sites, but you couldn't get it in the App Store on your iPhone, and you weren't spammed with ads for it every time you watch football.

It's not hard to imagine going back to that world; it wasn't that long ago. I used to support legalizing online gambling back then, but we've run the experiment and seen the results and it's past time to shut it down.