Once an access request is approved, it moves on to the fulfillment stage. You cannot cancel a request that has been approved. If this is a manually fulfilled access item (i.e. IdentityNow doesn’t automatically provision the access, an admin has to manually provision the access), then the only course of action I see is to complete the request, without actually provisioning the access, and leave a comment, if possible, that the request was not fulfilled as it was made in error.