Final answer:
Propranolol is the medication that would most effectively block sympathetic nervous system stimulation of beta cells in the heart, reducing both heart rate and contraction force as it is a non-selective β-blocker.
Step-by-step explanation:
The medication that would most effectively block sympathetic nervous system stimulation of the beta cells in the heart, thereby reducing the heart rate and force of contraction, is propranolol. Propranolol is a β-blocker that nonspecifically blocks β-receptors. These receptors are responsible for vasoconstriction and cardioacceleration, and blocking them allows blood vessels to dilate and keeps heart rate from increasing, thus improving cardiac function especially in patients with cardiovascular disease.