Based on what I have read, and only on what I have read, which is pretty much every article I could find, [ because so far I have not had to treat Ich myself ], it MAY be useful to remove heavily infested fish to QT. But you would still have to treat every fish.
If they are schooling fish, maybe the stress wouldn't be worth QT, but in any case, all that the QT for a few fish is meant to accomplish, when the main tank has to be treated anyway, is to reduce the parasite load in the main tank.
If one or a few fish are really heavily infested, when those Ich hit their reproductive phase, they'll add a lot of new Ich. If the other fish are only lightly affected, keeping the few who are really covered in Ich separate in QT might help reduce overall numbers in the main tank. It might be worth doing and it might be a waste of time, but it's something I have seen suggested a few times.
From all I understand too, you really must treat all the fish, because once Ich are in a tank, so long as there are fish for them to infest, there will be Ich.
The only other way to get rid of Ich in a given tank is to remove all fish, long enough for the Ich to die out. I can think of one reason you might do this.. if you had a planted tank with sensitive plants you didn't want to risk damaging with some med or other, removing fish to treat them in another tank is a way to skip treating the main tank.
You'd let the Ich die off naturally in the main tank. You'd raise the temp as much as was safe to speed up the Ich life cycle, and there are any number of articles that give you a good idea how long Ich can survive without fish at various temperatures. In cold water, it's a long time. It warm water, not that long.
Removing all fish means Ich will die out, as they'll be unable to complete their life cycle.
