The California Supreme Court has determined that a trial court has no authority to dismiss a conviction under penal code 1385 after probation has ended.  "At any time before a criminal defendant pleads guilty, receives a suspension of sentence, begins a term of probation, or, indeed, completes it, the defendant may invite the trial court to act in the interests of justice and dismiss the action against him. By the time the defendant has completed probation, however, the trial court’s power under section 1385 to grant him the relief he seeks has run out. The action against the defendant has ceased, the court may no longer impose a final judgment on him, and the court’s dismissal power under section 1385 has expired."

The full text of People v. Chavez can be found here: