How can I help my cat whose managed to scratch the back of her neck raw?

My cat had a small scratch on her upper back (right about where the scruff of the neck is) a couple of weeks ago but as it healed, she managed to reach. Now it’s a large hairless patch ( a few inches wide) with 2 nickel sized open sores that she can scratch with her back paws and can sometimes twist around and lick. I tried a tea tree oil spray designed for hot spots on pets that is supposed to be soothing but didn’t appear to be. I also put neosporin on it, which helped but just before it heals she completely scratches the scabs off. It seems not to bother her for a day or two, then she scratches the crap out of it.
Is there an over-the-counter product that I can buy that can keep her from getting at and scratching this spot? Maybe a hydrocortisone spray? At what point should I take her to the vet (which is expensive but will do it if necessary)? (I tend to over panic about pet injuries. When I get to the vet it turns out to be something I can take care of at home, that’s why I ask.)