Final answer:
Protamine is the agent that competitively inhibits the anticoagulant effects of heparin. Unlike aspirin, warfarin, and clopidogrel, protamine directly reverses the effects of heparin.
Step-by-step explanation:
The agent that antagonizes the effects of heparin is protamine. Heparin is a short-acting anticoagulant that is often used therapeutically, for example, in surgical patients at risk for blood clots to prevent coagulation. Protamine is a medication that can quickly reverse the effects of heparin, making it an antagonist to heparin's action. Aspirin, warfarin, and clopidogrel are also anticoagulants, but they work through different mechanisms and do not directly reverse the effects of heparin.