@Koi beginner ;
here's a trick you can try; catch all the fish and put them in a temp holding bin. If you have any structural pieces in the pond--rocks, tubes, even pots of plants, move them all around. Sort of like moving the furniture around. You can even move any aerators, add MORE rocks or plant pots, anything to disrupt the normal.
Then put the fish back in, letting the koi go first. You can even keep the goldfish waiting for a bit to give the koi some time to acclimate first/better. Then add the goldfish. Oh, and don't feed them for a couple of days ahead of time. Then, after they're all back in the pond, if they don't immediately hide, throw a few food pellets in and see how that goes.
I used to do something like this with my tropicals; established chichlids were established for a reason and they didn't like newcomers that didn't know their place and which territory they were entering so when I rearranged the furntiture, all territory was now up for grabs and new fish weren't as concerning as figuring out what happened to their world.
I don't have this problem now with my 'herd' of fish and I've never had my gf chase my new koi additions, but I also think when you have a LOT of fish, it's hard to pick on only one. Helps too to have a larger pond. Is LPS in your future????