The Vagal maneuver is a known way that can stop a racing heartbeat, although medication might be necessary for long term, as others have said:

Vagal maneuver and Valsalva maneuver (options, not all 3):

Strain as if having a difficult bowel movement

Rub the neck just below the angle of the jaw (which stimulates a sensitive area on the carotid artery called the carotid sinus)

Plunge the face into a bowl of ice-cold water