Owls and Hawks are natural enemies so by putting out an owl decoy where hawks are will keep and scare the hawks away from the area.
When a hawk sees an owl they will usually fly off away from the area since the hawk and owl are enemies and will usually stay away from each other.
If you want to keep the hawks away just place the owl decoy near the area such as a chicken coop etc and the hawks will almost always fly and stay away from the area.
You can also cover a chicken coop with wire on the top and put some owl decoys on the top of the chicken coop and it will keep hawks away.
Adding a scarecrow will also work to keep hawks away because hawks will see the scarecrow as an enemy and will fly off when they try to land near the area where the scarecrow is.