Travel to US.
Without a valid ESTA , visa, or Canadian passport you are not permitted to board an aircraft to the US.
So the Border agency have little or no problem in restricting travel from designated areas.
Could be I am oversimplifying but that is my understanding of the rules.
Canada has similar pre travel documentation requirements and if you intend to enter the US from a major Canadian airport
you have to pre clear US customs and immigration in Canada..