If a case is heard at the court of appeals level in a circuit court by a panel of three judges, and then en banc, and the en banc panel denies the appeal, the final level of court to which the petitioner may appeal is the Supreme Court of the United States.
The Supreme Court is the highest court in the United States and has the authority to review and overturn decisions made by lower federal courts, including the circuit courts of appeals. However, the Supreme Court is not required to hear every case that is appealed to it; it only hears cases that involve important legal issues or conflicts between different circuits.
Therefore, if the petitioner wishes to appeal the decision of the circuit court of appeals after an en banc panel has denied the appeal, they may file a petition for a writ of certiorari with the Supreme Court. If the Supreme Court decides to hear the case, it will review the lower court's decision and issue a final ruling. If the Supreme Court declines to hear the case, the decision of the circuit court of appeals stands as the final ruling in the case.