What term identifies a receptor blocker that prevents or cancels an action?

The term that identifies a receptor blocker that prevents or cancels an action is "antagonist." Antagonists are substances that bind to receptors and inhibit or block the action of agonists (which are substances that activate receptors). When an antagonist attaches to a receptor, it prevents the normal physiological response that would occur with the binding of an agonist.

In pharmacology, understanding the difference between agonists and antagonists is crucial because it helps in designing drugs that can either enhance or inhibit specific physiological effects. For example, in pain management, certain medications may act as antagonists to block pain signals, thereby providing relief.

The other terms listed in the options play different roles: agonists activate receptors, inhibitors generally reduce the activity of enzymes or proteins but do not necessarily block receptor action, and activators are typically used to describe substances that enhance the function of certain biological processes. Thus, the use of the term antagonist precisely fits the description of a receptor blocker that negates an action.