With great respect for Mr. Madmax, I don't think it's a ridiculous question at all. I recognize we don't have jurisdiction beyond the fence, but we do have a game management role.
I had it in a JV game with very few around in the stands except for a clearly-close-to-the-team parent who was kinda/sorta coaching the players informally doing this right behind a short cramped backstop, so about 3 feet from me and the players.
Isolated, rough and ready city field. Two very rough teams, though with excellent, development-oriented coaches. No admin for miles
Ridiculous and distracting. Same as relaying signs by someone in the crowd, and I think appropriate to stop.
I did address it. Was going to say something to the coach like "Is that a coach over there? If not, please ask your parent to stop signaling the hitter before we continue" and he would have done it, but I opted for the low-key direct approach. Its a neighborhood where there is a LOT of back and forth at the games.
"Hey man, do me a favor, can you not call out the catcher position before the pitch?"
"hey I"m I'm being polite!" (which come to think of it is a slightly odd response and indicative of someone who may have had similar discussions before)
"You are being polite, and I'm not asking you to move. Calling out positions is a little different and its distracting, and I'd appreciate it if you can stop"
He grumbled off and I heard the coach ask him what was going on a little later, parent said "yeah, I get it actually."
So, it worked. Which I recognize does NOT mean its the right approach. But I think it was right for this game.
My other/next step would have been to go to the coach and say I'm going to treat your guy as a coach since I've seen him interacting with you and the team a lot today, and I need you to tell him to stop this, Or I've gotta restrict both of you. Lets not go there. I'll hold the game till he stops, and you go talk to him.
Again, this approach felt right and in keeping with the vibe of the day.
Have at it.