I would have "your attorney" send them a notification that you have a video recording of them violating federal law by entering your mail box.
Maybe a reference that the letter should be treated as a cease and desist? Any further actions including slow drive by's to case the property will result in charges being filed?
Legal bluff are a bad idea. If you're going to get into a legal spat, just go full force straight off the bat without any warning.
If it's American hating leftists who get offended by these flags, they deserve to have the book thrown at them.
They don't tolerate your existence, why should you tolerate theirs?