How to get rid of QLD itch?

I don't think there really IS any way to "stop" it, unfortunately; the best you can realistically shoot for is reducing symptoms to whatever degree possible. If you are absolutely CERTAIN that's what it is (there are a variety of other itchy-skin conditions sometimes mistaken for it)

(Americans may be more familiar with Queensland itch under the name "sweet itch", it is the same thing, it is an allergic reaction to midge bites).

Ideally sell the horse to someone who lives in a region where the problem midges are infrequent or absent. Realistically, you can sometimes manipulate turnout time and stall screens and repellants to reduce the amount of exposure to the midges. Some people roach the mane of a horse with bad sweet-itch, not that it really helps reduce symptoms but it can make things look less raggedy and easier to put flyspray all over. I'm not saying it necessarily helps, I'm just saying, a number of people do it.

There are veterinary treatments that can be tried but they are not without side effects and risks, and don't necessarily always help anyhow.

It's not a great thing to have, unfortunately. Hopefully your horse actually has something else more fixable; or you can find some way of keeping her that she doesn't get bit so much and thus doesn't get so itchy and scabby.

Best of luck,

Pat