If they are from reputable institutions, contact the institutions' ethics/compliance section. It is up to them to investigate. Clearly this only works if the institutions care about their academic reputations and the damage that having their people participating in paper mills does to them.
Note that a high rate of output is not automatically fraudulent. I have heard recently of a case at my institution where an academic was publishing at a seemingly ridiculous rate. They were investigated, and were one of those rare cases where it is legit. Lots of documentation demonstrating that they are indeed being that productive with their team and pumping out huge volumes.