Valve has issued a statement: "From time to time a mistake can be made and one was made by prematurely issuing a copy of War Z for sale via Steam. We apologize for this and have temporary removed the sale offering of the title until we have time to work with the developer and have confidence in a new build. Those who purchased the game and wish to continue playing it via Steam may do so. Those who purchased the title via Steam and are unhappy with what they received may seek a refund by creating a ticket at our support site."
If you want a refund, now's the time!