If they are going that far out of the way to screw over someone for their benefit, I don't think it would be immoral to save their information and publish it at a future time so that it can't easily be traced back to you.
Obviously, you want the honest customers to be able to get faster better service by being able to tip up front. I don't think that it is wrong to be able to take back a tip if there is legitimate reason to do so for poor service.
I see absolutely nothing wrong with not adding a tip in advance on the order, and potentially having to wait longer for the order. That is honest and transparent. But taking back a tip for no other reason than to get the order quicker for free and hurt the rating and potentially the livelihood of the person doing this work is completely different and unacceptable scummy behavior, and should be punished.
An easy solution on how to handle the people who abuse this system could be to allow customer accounts to take back tips if they deem necessary, but warn them if this pattern is consistent that their ability to take back tips will be disabled. A complicated part about this might be controlling the ability to make a new account using a new email address or phone number. Someone who will premeditate a tip reversal is probably also likely to create a new account once their tip reversal feature is disabled. From instacart's perspective, you would not want there to be a very difficult barrier to become a new customer.