(For your specific problem:)
From what I can see, the accountability is just in the Spirit of the Game (i.e. 1.10 and 1.11). It technically doesn’t say anything about *making* a call, but making sure everyone understands that Non-Players shouldn’t influence call discussion could help.
I personally think it’s fine if people on the sideline remind people of calls, as long as it’s not ambiguous like Amaury pointed out, and as long as it’s not “malicious” but that is also already very clearly addressed in the rules.
(For the rules:)
Adding something for accidental or mistaken calls that didn’t come from a player could be useful. Something along the lines of:
“If a player reacts accordingly to a call they believe was made on field, but which was not made by someone who would be allowed to make that call, they may call a violation leading to a stoppage of play.”
And then have some decision tree with whether it affected a play, either leading to the disc being returned to the last thrower before the call or just being checked in again at the current thrower.
People on the sideline making ambiguous sounding calls should just be treated like a Spirit violation I think. It should be talked about before and/or after the game or during a spirit-timeout if it’s happening a lot during a match.