Anything oily, vaseline, baby oil, even veg oil at a push. I know some will mention sunburn but the tail area isn't too vulnerable unless he/her has rubbed it raw. You need to stop the munching barstewards from getting to the skin. Garlic if you have cloves you can crush into a paste.
For a repellant, Avon Skin So Soft, Neem oil, garlic, citronella, eucalyptus.
I got to the paddock tonight to find my Missy had a bit of a bog brush look going on. Friend 'did' her yesterday . I stress how important it is to really rub everything in to the skin and make sure she is well covered. She has been well slathered today.
Thats what you need to do now before he/she gets into the vicious cycle.