So far that is unlikely to stop her eating. Being separated from the others is more likely to make it go off its feed!
Give it a wash (our vet recommended a strong solution of Daz - not Ariel or Bold, gotta be Daz - and I found that as good as using purple or blue spray. Blue spray not much use unless it's infected anyway, as you point out.)
There is orf about on your land, clearly, so the best things you can do now are:
- keep the lambs away from anywhere that has thorns or thistles or anything that will pierce skim : that's how it gets in
- watch ewes with orfy lambs, and look for signs that any lambs are hungry and pestering a ewe, because orf can spread to - and from - the udder, and if the lambs are sucking hard because she doesn't have much milk, that can break the skin on the teats. Feed ewes if the lambs are pestering them, to reduce the risk of broken skin and orf
- consider vaccinating all your lambs. Scabivax is a live vaccine, easy to apply (but do read the instructions and follow them), very effective. Because it is a live vaccine, you shouldn't use it unless you are sure you have orf on your land - but you clearly do so it is an option for you.