if you want people who shop at walmart to know you do not like a specific product you can visit their website & leave a review. the employees have no (power) to put up warnings---unless their is a re-call...an employee would face loosing their job if they did what you asked & in this economy not many are looking to loose their job over a can of spray.
posting reviews is really the best way to let others know there is a problem---and of course reading reviews when checking out new products will save you some time, money & heart ache in the future.