There must be a centralized organization that must fund the community policing of this. So you get donations from the community to fund a global centralized initiative to monitor and report on scam sites. You can use these funds to pay for the hosting of a website that lists the bad apples and also the legitimate sites.

You can also reward people that are willing to spend time and resources to investigate these sites and then to bring proof, if these sites turns out to be scams. A lot of sites might look legitimate at the start and then they turn scam at a later stage.

Most Ponzi schemes will honour all pay out in the beginning and only later run with the money.