What herb or pill can you take to stop svt?

Not herbs. Supraventricular tachycardia and other cardiac arrhythmias need to be evaluated and followed by a physician -- ultimately, probably a cardiologist. Certain medications and/or substances you're taking can cause or exacerbate this too. Though magnesium has helped some people with svt, I wouldn't rely solely on this or any herb. Your primary care doctor is a place to begin, and go from there.