Say it out loud or be silent but stop mumbling
I think that's the wrong approach. Sure, for trivial stuff it might be fine, but for stuff that needs moderating there are problems:
1. Perhaps the complaint isn't valid (misunderstood rules or something). If that had been brought up in public there would be arguments, probably pedantic and leading off on tangents to other arguments). Afterwards, the complainer would have his card marked by some subset of participants. On the other hand, a non-public complaint can be settled by a quiet word from the mod setting out why it's not a valid complaint.
That doesn't stand up to scruitiny. If it isn't valid then an open discussion will clear that up. If it is valid, then an open discussion can give the perp an early warning that peers are applying pressure.
2. Perhaps the complaint is valid. Bring it up in public and... well, repeat of 1. I am sure. A non-public complaint can see the mod have a quiet word with the transgressor, and it's more likely to be settled then because there would be no big ego losing face as there would be in public.
That doesn't stand up to scruitiny. If a transgressor realises other people think they are transgressing, then they may speedily modify their behaviour due to peer pressure. If they aren't warned until "later", then the behaviour might have become more entrenched and so there's more face to be lost.
We used to get pop-up single-post vendors flogging stuff, and a following trail of posts telling them to push off or questioning what they're about, etc. But we don't get that now because instead of whining people hit the compaint button and a mod just goes and deals with it, no fuss. Sometimes it's a pukka post and the mod leaves it, and the difference is there isn't a long thread of complaints to go with it.
That isn't the case under discussion. When there is a
clear cut case forum rules
deliberately being broken for commercial gain (or equivalent) then I warn the mods so they stop on such perps.
Basically I think it cowardly and deceitful to "go behind someone's" back, and "complain to mummy" that "I don't like what Bobby Droptables is saying". Or, as in too many states, tell the Authorities "I suspect Mary Jo is a communist/capitalist enemy of the people" (delete as appropriate). When that happens, Mary Jo isn't told of the
details of the accusations, so they can't set the record straight and can't discredit the evidence amd/or accuser.
There were echoes of that in the recent debacle, and they have left a bad taste in some people's mouths. That's partly why some of the departed aren't keen to return.