If you have some white cayenne pepper you mmigh try sprinkling that over the flowers, but there are also sprays you can get that have some type of hot spice and also soething on the order of wax so it keeps it on the flowers, that could work better especially if they are nipping them from the underside of the flower heads.
Rabbits will do this too! I have Black-eyed Susans that they used to take the tops off!
I have had good sucess with SEVIN powder- it is an insecticide most all home/plants stores carry it. I get it in a bag and put some into a shaker- then sprinkle it on the plants (on a non-windy day!) re-do if it rains.
Seems like most animals do not like the smell/taste and will leave your plants alone.