How do I tame my Amaranths and Pansies?

A shortage of direct sunlight combined with fertile soil is probably the reason that the plants have gone berserk . strong direct sun helps prevent tall gangly growth , if you prune the plants you will possibly be cutting off the flower buds and so you may not get any flowers , try tieing them to a dowel or something else and just hope for the best. Next time look for seeds of plants that like low light conditions and see if they do better.