The only thing that works for me and it doesn't work terribly well is constant harassment. I run them off when I see them. I have a dog but he must be short-sighted as when they are still they blend in and he doesn't see them from the house. Use your dogs to your advantage if possible as they are very effective harassment for the GBH. We also use airsoft guns (semi-auto) as a deterrent both due to sound and BBs landing around them. No way we could hurt them with airsoft.

If the GBH are determined like ours are they just roost in a tree and 15 minutes come back at which time we have to start over.

Then if we are too successful then I'll drive in around midnight and they will be there happily harpooning away in the dark.