Too late by: Jacki Cammidge, Certified Horticulturist
As sunflowers are an annual (seed to seed in one season) you may as well just pull them up as they won't amount to anything now. Just learn from it, and get onto any problems sooner.
Plant something else that will give you some fall color as it's already the end of August. Pansies or some other fall flowering plant will work well, if they like your climate.