Not specifically sunflowers, just plants in general. If you go into a spruce wood, you will notice that not much else grows there. Especially not plants hungry for nutrients like sunflowers. I don't think they are like yews, which put out chemicals to discourage plants growing around them. I think they are just very effective at taking up all the nutrients and water for themselves, and being shallow rooted don't leave much left for other things. They also cast heavy shade. Also decomposing spruce needles make the soil very acidic.