Whac-A-Mole, not Whack-A-Mole, is the name of the American arcade game that involves whacking erratic plastic moles with a mallet. The correct spelling of Whac-A-Mole may seem an esoteric point, but the word has entered the language as a metaphorical term for something that keeps popping up no matter how many times one tries to get rid of it. If we wish to honor the word’s source, the only correct spelling is Whac-A-Mole.
Still, getting writers to use Whac-A-Mole (or even the lowercased whac-a-mole) is probably an uphill battle. Whack-a-mole is about three times as common as whac-a-mole in current news publications. This writers use it, for instance:
To his detractors, his policy seems more like whack-a-mole. [Boston Globe]
Encountering online ads is like being at a carnival and playing whack-a-mole. [Washington Post]
[L]awmakers are responding to complaints from business owners who are weary of playing whack-a-mole with varying regulations from one city to the next. [New York Times]
But not everyone gets it wrong—for example:
The PBDEs … sensitized the public to the “whac-a-mole” problem. [National Resources Defense Council]
Tuesday’s action was the latest in a European debt crisis that is closely resembling the arcade game Whac-A-Mole. [Forbes]
[T]he administration’s approach to these different situations where values and interests collided came to resemble a giant game of Whac-A-Mole. [Wilson Quarterly]
Your spell check will probably disapprove of whac, but spell check is often behind the times.