How to stop a heart attack?

Revascularization. The only way to stop a myocardial infarction is to re open the obstructed or nearly obstructed coronary artery. That is why if the coronary artery can be reopened within minutes after the obstruction the early and long term survival is improved.
Acute or chronic? If you're having one right now, please go to the er. If you mean long term, don't smoke, do exercise, stay normal weight, have your blood pressure and cholesterol checked and have treated if indicated. If you're male and >50, take Aspirin (65 for women). And finally, please choose the right parents.