I don't know that petroleum jelly would be a good idea. Not only could it just make a big greasy mess, but it may just encourage them to groom the area even more to clean him off. If you can find that bitter apple spray at a petstore, and it is safe to put on the skin, that may work. The problem is that anything nasty enough to discourage grooming, may not be safe for the skin or for ingestion, and anything less than that may just encourage even more grooming.