Adrug molecule that inhibits or blocks a neurotransmitter's action is called

The correct answer is "antagonist".
A neurotransmitter is a chemical substance of the human body responsible for neurotransmission. Once released from the presynaptic neuron, they bind to specialized receptors located on the postsynaptic neuron. This binding triggers a specific response at the receiving cell.
An antagonist drug reduces the effects of a neurotransmitter by binding to the specific receptor and preventing the binding of the neurotransmitter.