If you are referring to a paid app for which the user got a refund, I don't think there is anything the app developer can do. I assume that such an app will no longer be available to the user on their device but I'm not certain. I'm pretty sure the app will not longer be available on other devices owned by that user.
If you are referring to an IAP (non-consumable or non-renewing subscription) then the app can ask for a refreshed receipt and check it for a cancellation_field. But this requires that the user log-in to their App Store account and is quite awkward. What will you do - require every user to referesh their receipt each time they open the app?
And yes - the money will be deducted from your account.
The good news is that 'cancellations' are very rare. And even rarer is the user who cancels and then continues to use the IAP. So my advice is to ignore this concern, however legitimate it may be.
And yes, sadly enough, the user will still be able to use the app on their device, even after the refund (for the app itself. I don't know what happens to IAPs).
Apple doesn't decide on their own...the process starts with a user flagging a purchase/requesting a refund, or in the case of upgrading a subscription:
"Upgrade. A user purchases a subscription that offers a higher level of service than their current subscription. They are immediately upgraded and receive a refund of the prorated amount of their original subscription."
If, however, you mean that the process doesn't include your vote, keep in mind that Apple runs the show and users come before devs, and yes, when refunds are given, your revenue is affected, not just Apple's.