Repeated or continuous flushing with your garden hose. Remove the engine side of your heater hose return line. insert garden hose in rad with the water on just enough. also open radiator drain, start engine let idle and go make a sandwich.
Flush out with garden hose. Then wash out with TSP, a common household cleaner. After running the car for ling enough for it to get completely up to temperature let it cool and rinse it out several more times. Just remember that this will also wash all the stop leak out of the holes you were trying to plug.