I hang old CDs (and DVDs) from the wire on fishing line, the slightest breeze sends them spinning. I've seen hawks (and eagles) eye my coop and run, but never try for it. It also stopped the scrub jays, Stellars jays and magpies from testing to wire to see if they could get in to get at the chicken scratch.
Although lately my favorite hen has been jumping to peck at the lowest-hanging CD, which she can't quite reach....
I might also try a garden hose and water, if you're close enough to spray the hawk and not the chickens.