There are nuances between jurisdictions, but while a US judge can reject a plea deal they generally cannot reject a guilty plea. A judge cannot force a trial between parties that don't want one. And a trial isn't really possible where the parties agree the facts. The judge can certainly impose a sentence different than that which was agreed between the parties, but any such punishment would have to be based on the agreed facts/charges/pleadings. If the prosecution decides to only charge a lesser offense, the judge's power would be limited to the maximum penalty associated with those reduced charges.