I did some further research into this and I found out that upon completion of the diversion program my case actually was not filed. The county in CA where I did pretrial diversion, Santa Clara County adopted a new policy on petty theft diversion for first time offenders in July 2012. If the diversion program was completed successfully then the case would not be filed in court.
I confirmed this by looking at the Santa Clara County case index of criminal cases filed in court, and my name/case number had no history available.
So if my case is not even filed in court, can I say no to a deferred prosecution?