Isaiah Thompson is the poster boy for a subway ban — a menace that the system needs to be able to keep out.

This was true even before Wednesday, when he slammed a woman head-first into a train in Brooklyn.

At 23, his rap sheet over the last two years already includes at least 18 transit-related offenses, including exposure, an attempt to throw another female straphanger in front of a train, slashing a man at the Jay Street-MetroTech station and causing more than 750 train delays by pulling the emergency brake.

He also likes to subway surf and, as he puts it, “cause mayhem” and “inconvenience people” — with considerable video evidence showing him doing just that.

He shouldn’t be walking the streets, and he certainly shouldn’t be let back into the subway system — where it’s far too easy for a single miscreant to endanger countless others.

Gov. Andrew Cuomo said Friday that he backs legislation to ban repeat offenders from mass transit: “There ought to be a law that says you don’t have an unlimited right to sexually assault people in the subways, you don’t have an unlimited right to violently attack people in the subway system.”

More specifically, “I think we have to have a system where a judge can say ‘enough is a enough’ and limit a person’s right to go back into the subway system,” said the gov.

State Sen. Diane Savino (D-Staten Island/Brooklyn), in fact, got just such a bill through the Senate last year — but the Assembly refused to even consider it, insisting (as she puts it) that “the current penalties are sufficient.”

They’re plainly not, as Thompson proves.

And, worse, they’re about to get weaker — since another reform is eliminating cash bail entirely for most offenses come Jan. 1.

The city might be able to handle Isaiah Thompson by invoking Kendra’s Law, since he seems to have mental health issues that pose a clear public-safety threat.

But unless Cuomo can budge the Assembly, it looks like the subway will only be growing more dangerous.