Yep. The BLM has programs to take care of wild horses on BLM land. They do a periodic round-up and they have corrals and equine-handling equipment. A ferrier I talked to once had witnessed some of the activities. They restrained the horses in some kind of a chute and thing like a calf table only full-size, and one by one trimmed their hooves using a power circular grinder. Very fast. Requires some kind of very effective restraints too. Had to do it on a regular basis. I suppose not as often as a captive horse because these wild herds are running around on rocky soil wearing down their hooves.
So I simply cannot believe that castration is not the easiest way. Look up elastrator, it's a device, one of the ways that cattlemen castrate calves. Apply an elastic band that cuts off blood supply to the testicles and after a while it just falls off. No blood, no surgery.