Just removing the wilted bloom is usually sufficient for petunias. I just pull them off and don't bother breaking the stem. The purpose of removing the spent flowers is to prevent the plant from going to seed, which is sometimes referred to as "setting seed."
The goal of a plant is to create seed, and once it achieves that, it usually stops producing flowers. Removing the old flowers keeps the plant in bloom longer.
"Isn't it enough to see that a garden is beautiful without having to believe that there are fairies at the bottom of it too?" - Douglas Adams