Don't let them go to seed. Amaranth sets millions of seeds which can live in the soil for several years. Pull it as soon as you see it and in a few years you 'may' get rid of it.

George

On the other hand, it's delicious. :)

I agree. Pig weed is much better than spinach. Easy to identify with a dusty look on the back of the leaf. Best for eating if it's under 6" tall.

Pig Weed, "Amaranthus" species, is often a edible plant and many people do use it as a cover or green manure crop. As you have discovered this is one of many reasons why purchasing "soil" from some place else is often a bad idea although yo udid get one major benefit and that is a source of organic matter to put into that soil. Do not pull them out of the soil but instead till them in.