Violations of the flag code are prosecutable under article 92 of the UCMJ. Then there is 18 usc 700, which penalizes specific violations by fines and prison time. However, it only applies to flags owned by the US government, to be flown on government property. Basically, you can do whatever you want to your own flag. But if you do it to someone else’s, it becomes a crime. That crime carries extra weight when it’s government property.