How does the FBI have the power to impose a lifetime ban.
I doubt that they do. But they would refer their findings to a court, or to the Homeland Security Agency or someone else who does.
This reminds me of the incident where a university student in Texas heckled W junior, shortly after took office first time. He promptly passed some law that insulting the commander in chief was a punishable offence and had the kid arrested retrospectively.
First, the president (whoever) doesn't pass a law. Congress does.
And I doubt the truth of the incident described, in any case. That would be what's called an "ex post facto" law, which is, I believe, prohibited by the Constitution.
Do you have any reference on that alleged incident and that alleged law and the retrospective arrest?