Even launching a ping pong ball at a hawk or owl will open the door for fines and prison time. Nothing can be projectile towards a Federally Protected Animal.You could try a pellet gun. Although there are problems with using that, hit them when they fly and you could hurt their wings or bust their big eyes. If you decide to use a pellet gun, use it carefully, while the owl sits perhaps. Even if it is risky, it would probably keep the owl away for a good long time.
Publishing illegal acts on the web is not a wise thing to do. This site is not immune to Federal Law Enforcement as well.
I have gave enough fair warning.
It's like teenagers beating the heck out of some poor kid at the bus stop. then posting it on Facebook. When will people learn? Always after the handcuffs.