First of all, the only thing I put into my water is dechlor. The string algae, other than being a pain for few months is actually beneficial to my ponds eco system. My koi spawn in it and the string algae provides a safe haven until I can harvest the eggs. Those fry that hatch in the pond use it for food and refuge.
This time of the year is when the spawn occurs and the string algae's life ends. A perfect world for me. I would not try to stop it for the aformentioned reasons.
Many here, who do not raise Koi are using barley and peroxide. Some are a little more sophisticated and are using other products that you may want to study a little chemistry before using. As for the snake oil sold by fish stores, I wouldn't use it if they gave it to me.