Ignition timing will need to be corrected first and your Geo Prizm given a second smog test. Incorrect ignition timing alone, can cause high NO emissions.
If this Geo Prizm is a California emissions certified GSI model it will also be equipped with an EGR valve; which if not function properly can increase NO as well.
Using higher octane gas will not remove carbon build-up. Follow the link below to learn more about NOx failure.
What causes high NOx?