I didn't receive a purchase notification from Google Play (due to a fault on my own platform) so, correctly, after 3 minutes the purchase was cancelled. That's fine.
However, the platform continues to send me notifications but complains (with a message 400) that the acknowledgement I send is faulty because "The product purchase is not owned by the user."
I can't cancel the purchase through the Developer Console, because it's already cancelled.
How can I stop the purchase notifications?
It's not a satisfactory answer, but an answer is "don't worry too much about it, it will only run for a week".
Unless the Developer Console helpdesk have actioned it for me without letting me know, it eventually times out. The notifications have stopped after about six days and twenty-three hours.
Let me know if you find any different :)