I have used old gill net made from very light monofilament line. They hit this net at full speed at the end of their dive. It is definitely a physical barrier!!! Once at a friend's house, we witnessed a hawk fly smack into a chain link fence trying to get at his turkey poults! I guess they get so focused on the intended meal....
We have strung crisscrossing single strands of monofilament fishing line over an area where a neighbor lets his chickens forage (about 60' X 100'). He used trees and erected conduit and PVC poles to attach the fishing line to. Neither he nor his wife have actually witnessed any hawks hitting the lines. However, they don't even hang around any longer. We assume from the poles getting knocked around somewhat, and the hawks no longer perching nearby or, flying overhead is some indication they must have hit the strung-up lines.
The poles weren't the nicest-looking things but, if in fact they are responsible for the disappearance of the raptors, those neighbors would have set up triple the numbers of poles.
We live smack in the midst of large commercial fisheries and old gill net is easy to come by. If you use it, make sure you stretch it out tightly. If you don't, the hawk or owl will get all tangled up in the loose webbing.
New gill net can be purchased here for around $10.00 a pound. The net can be as deep/tall as 6 feet to 14 feet. Imagine a nearly invisible wall 6 to 14 feet high! Call these folks to find out how many feet there is to a pound.
http://www.louisianasportnets.com/4mono.htm
I have no affiliation with any net or trap company.