Signs like these hardly ever work. It's usually just as easy to “error proof” this to make it impossible to place something on the top shelf. Signs are too easily ignored.
If signs are used, managers need to make sure everybody understands WHY things can't be placed on the shelf. Is it because of fire code? For employee ergonomics?
Managers need to follow up and pay attention to see if signs are being followed (or, better yet, making it impossible for people to put things on the top shelf). For example, the top shelf could be removed from the rack altogether without affecting the structural stability of it.