Anyway, thanks for the input, guys. I've never had a pitcher try to undo an intentional walk. Usually, they drag out the process so much when deciding whether or not to do it, once they finally decide to do it, by golly, they're going to do it!
And since their announcement of intentionally walking the batter automatically made him a batter-runner, I did not feel right in allowing them to hit the "undo" button.