The family of Duane Christian want to sue the province's police watchdog over its investigation of the shooting.
But the high court has rejected their case.
As usual, the justices gave no reasons for their decision.
The family filed a $2-million negligence suit against Ontario's Special Investigations Unit after it cleared police of wrongdoing in the death.
While lower courts backed the family's right to sue, the Ontario Court of Appeal dismissed the lawsuit.
The family then sought leave to appeal to the Supreme Court.
Christian, 15, was shot by police while driving a stolen minivan in the parking lot of a high-rise apartment building.
The policeman who fired the fatal shot told an inquest he had no choice because the teen was trying to run down his partner.
The family said the SIU was incompetent in its investigation and argued that allowing families to sue in such cases would promote better investigations.
The appeal court held that the unit owes a duty to the general public, not to victims or their families.