Final answer:
Prostacyclin (PGI2) does not inhibit platelet aggregation.
Step-by-step explanation:
The substance that does NOT inhibit platelet aggregation is Prostacyclin (PGI2). Prostacyclin inhibits platelet aggregation and promotes vasodilation. On the other hand, Aspirin inhibits platelet aggregation by blocking the enzyme cyclooxygenase, which is involved in the synthesis of thromboxane A2, another substance that promotes platelet aggregation.