no idea on the background on all of this, but this all comes down to what can and cant be proved, with or without evidence.
but if the scams have in fact been committed then its fairly simple.
contact local police
report the crime, provide evidence, information, reciepts or some form of proof of purchase (or evidence that reflects a scam)
police will investigate, collect evidence
if substantial evidence exists then police raise search warrant to internet provider
internet provider provides subscriber details to police
police conduct investigation with subscriber information and investigate futher
that is, assuming you dont know who the person is
obviously its easier if you have identity information