In some cases, it is quite possible to wash off hairspray with water. But this depends on the spray’s effectiveness and if it has water-proof qualities. Additionally, it also depends on how strong the hold of the spray is and the amount of the product applied. A hair spray with a low-level hold might wash off easily with water while a strong hold might require the need of a cleansing agent like a professional shampoo.
It is recommended to use a clarifying shampoo to remove the hairspray so that there is no product build-up.