Are there sny over-the-counter potassium-channel blockers?

No: Because this would be extremely dangerous, as there potassium channels are quite ubiquitous (meaning in almost all cells). And the common potassium channel blocker are class iii anti-arrthymics (amiodarone, dronedarone, ibutilide, dofetilide, sotalol, vernakalant, and azimilide) and they can cause qt prolongation which can lead to fatal ventricular arrthymias. So it wouldn't be wish to have otc.