I just remembered, my grandmother used to swear by leaving hedge apples around the barn to keep spiders out. She milked cows most of her life so maybe she was right.
For years I called them spider balls till I found out what they were really called
I agree that they are usefull to have in the barn, but they give me the MAJOR creeps. If you want fewer spiders, you need less flies. A clean well ventilated barn has fewer bugs. I scrub our buckets every day with soap and water, keep feed in tight containers, sweep thoroughly, put down a mild and animal friendly disinfectant on the floor to keep dust down and help to discourage flies. I knock down any webs under a certain height ( me with the broom above my head) daily. It sounds like a lot, but it works and becomes part of your routine.
Plus, folks will notice what a lovely clean barn you eep, and your horse is sure to appreciate it too!