Prevention for scratching in the upper thigh?

Get to the cause: The question is why are you scratching. You will only stop once you learn the reason you are itching, and treat it. Most likely is a heat rash or fungal infection (commonly called jock itch). You can buy some over-the-counter antifungal cream for jock itch and apply it to the itchy area for a week or so, and see if it helps. If you're still itching, see your doctor.