Hairspray on your walls and mirrors is quite a common problem. However there is not an easy solution. Hairspray is lacquer and therefore difficult to remove.
I have had a tip sent to me that states that you can remove hairspray with shampoo on a damp cloth. Then wipe again with a clean damp cloth. It is worth a try. If not then on to the more dramatic measures below
Really it depends on the surface that you are trying to remove it from. If you are trying to remove it from a NON painted surface like a mirror or tiles then it will come off quite easily with rubbing alcohol (Isopropyl alcohol). Simply wet a cloth with the alcohol and rub off the hairspray.
Rubbing alcohol will strip the paint away from a painted surface. So if you are trying to remove the hairspray from a painted wall or similar then the only way is to re paint. The hairspray will not come through the new paint however you may want to give the area a light sanding before painting so that you have a smooth texture to paint.