I totally understand why Ubuntu shouldn't be supported here; I agree with that.
I also understand why other OS might stay.
But i also understand why guys ask about ubuntu here. (For example because they bought the hardware here; they like the guys here and want to share other ideas/projects on other topics here, ...)
But i think there is one small misunderstanding. Guys don't want support by RPF if they ask about ubuntu. They only want to talk with other users here.
I think it is a bit similar to Apple. Apple users also ask about windows stuff in the Apple community. They don't expect support by Apple itself (maybe except of the bootloader. BTW: Anyone knows if Microsoft pay Apple for that support?); they only want to ask the community.
So why don't give them just a forum. At a a sticky note that RPF support is not available in this forum. So you don't wast time in supporting; you don't waste time in writing that the world is unfair; the RPF will earn more money, because more guys will buy a raspberry (since there is a lovely community that helps each other, ...); ...
It is a little bit like in a restaurant: If a visitor buys a meal, but drink his wine from an other store. That is bad. Maybe it is like this. In that case lock the forum.
But maybe it is like a visitor that buys meal and drink, but smoke his cigarette from an other store. hmm... In most countries (if smoking is allowed) this is normal, nobody complain and say: You are only allowed to smoke here if you buy your cigarette in my store.
So i think you might loose a lot of customers if you tell them they are "bad". I fear it is like the inns here. Some years ago there were so many inns. But then law disallowed to smoke in the inns. I understand why that is correct; I myself like it, since i am not smoking. But i also saw the result of that law: a lot of inns didn't got enough customers anymore; so most of they are down and out now.
If a customer talks in the inn, he also don't expect an answer from the bar keeper. He only talks to other guests. Of course the bar keeper can answer if he want, but in normal case they don't do that.
Difficult. I know. But i think one of the upper solutions will be much better then continue "fighting", since "fighting" means only loosing (on both sides).