If you are using Inquisit 3 (which I assume, because you posted in the Inquisit 3 forum), this is not possible: Inquisit 3 does not have the capability to register key releases.
Inquisit 4, on the other hand, does have that capability. To define a key release as response, you prepend a minus-sign to the respective key's scan code.
/ validresponse = (-57)
would consider the release of the space bar a valid response.