I need help with my horses tail rubbing please!?

My stallion likes to do that. Normally b/c his tail bone is dirty and he wants it washed. Once I wash it he is fine and stops doing it until he decides he wants his tail washed again. I also find that MTG really helps too. I would suggest giving his tail a good wash all the way from the base to the end concentrating on the bone area. Deep washing and a very very good rinse.