There are a few different types of seed that grackles don't seem to like, including safflower and canary seed. Some people also report that grackles don't like sunflower seeds with the shells still on. It's worth experiment. More specifically, some research has shown that red-winged blackbirds prefer sunflower seeds that have been hulled, while common grackles prefer unhulled sunflower seeds. So if you're trying to attract one species or the other, you might want to try offering both types of seed.