The "people" that use the ponzi sites are just going to be sock puppet accounts of the owner that idea will not work.
I suspect this has already occurred - but if there is a limit set on when the account was registered, it may help.
Worse case, my idea fails miserably and I remove the ratings with a mention that there is a severe risk of being scammed due to dishonest site owners. Currently I'm willing to give them benefit of the doubt.
I've removed the one site from my list already since the member was new with a limited history and posting only about Ponzi sites.