To be honest, I do not understand the police and prosecutor's office's reasoning here, because all you need to charge a case is evidence that a crime was committed. It doesn't really matter which party was the victim here. Unfortunately, you cannot force the prosecutors to charge and pursue a case, even if it has merit, because they have discretion to choose what cases to pursue. Depending on who you talked to at the prosecutor's office, there may be more senior individuals that you can try to talk to (the supervisors perhaps, unless you've already spoken to the highest person in the office).
If the case involves the federal government in some way (say for example that the form that was sent in crossed state lines) it is possible that you could report it to the FBI instead. They don't have jurisdiction unless there is some federal element involved, but you can try to see if it is something they can pursue:
The only other place I could suggest going other than civil court is to contact the Attorney General
's Office. While it involves criminal conduct by a private citizen, since there is a business involved that is disbursing money based on alleged fraud, the AG's office may be able to get involved or to lean on the local prosecutor's office to perhaps pursue charges:
But if those don't work and you have to deal with your local prosecutors, unfortunately they ultimately get to decide what cases they pursue or not.