The solution is simple: Stop your dog scratching.
However, implementing the solution is the challenge.
Basically, every time you're with the dog you want to remain extra vigilant to make sure that she doesn't scratch it, distract her with something.
If you simply don't have the time to hawk eye your dog (as most don't), then it looks like a lampshade collar would be your best bet (they're available at most mid-size pet stores), which are something like:
My mother's Bichon had one of these in the past when she had a cut on her paw to stop her from licking it and breaking the scab and it is the only really practical and effective way of stopping your dog from scratching the scab.