Yes squirrels and chipmunks can be destructive in a yard or garden. Have you tried bird netting. It can be put on with hoops like the PVC pipe people use for hoop houses and the edges buried in the soil. When the sunflowers are too big to be broken off or dug out you can take it off. By then maybe they will have gotten the message

We used it to protect our strawberries from cedar wax wings. They would fly over en mass every evening and decimate our berries. We put it on and it worked for a day or 2 and then one brave soul crawled under it and was trapped. The birds sent up an awful racket so I went to investigate. I managed to free it amid dive bombing and there were no more evening visits

Dowlinggram - That was going to be my next step - the bird netting. Right now I am using the only piece we have on the strawberries. I don't have much longer to go until I can move it to the sunflowers. For now, I guess I'll leave the hardware cloth there until the netting is available. Very glad to know that it worked for you. I knew there had to be a way to grow them. Thank you. :-)