No there is no statute that oversees a prosecutor's discretion. It's actually (and UNFORTUNATELY) the other way around, and contrary to common sense, that as long as the state actor operates in what he BELIEVES to be good faith, he is operating with prosecutorial immunity. It all goes back to U.S v Leon, which not only built in personal safeguards relating to law enforcement, prosecutors, and yes, even legal-clerical workers like clerks and judges, it also allows for admission of evidence which, otherwise would have been suppressed, so long as the belief that it was gathered in good faith is articulable. Here, it's not like the DA is dropping all of the charges, he's dropping the ones that he realized (and which a judge would have clearly ruled pre-trial) that he would never be able to prove the elements of beyond a reasonable doubt. The gag order was issued by the judge, not the prosecutor, so all relevant actions, including the amending of or dropping of charges, are thereby gagged.