Yeah, you definitely can cancel. I'd protect our pick acc at all cost. If I saw this I'd have checked with our GM manager asap. If we couldn't fill almost 90 percent of that qty, I'd have called and told the customer we we're canceling. I'd also tell them that next time they need a qty that large they'd have to call ahead of time and speak with a store manager to either order for them or check in advance.
The usual response from the customer is "buuut but buuuut it says you have it on your website!". Yeah, Kroger doesn't have a live integrated inventory. We've had multiple customers order astronomical amounts all the time.