Is there a single word meaning "something which stops something"?

A trigger is something which causes something to start, e.g. caffeine and stress can be triggers for palpitations. Is there an opposite noun, a single word meaning "something which causes something to stop?" E.g. "Exercise can be a [thing which stops/mitigates/reduces] palpitations."

The scientific biological terms would be:

inhibitor (noun), inhibit (verb)

or in pharmacology (as you mention caffeine)

antagonist (noun), antogonize (verb)

although this latter tends to be used in relation to specific biological components, rather than an overall process.

In general speech there are several verbs that you could use, but perhaps fewer nouns. Possibilities that occur are:

block (on)

preventive (against)

But if you have heart palpitations I would see a doctor before exercising.