"Stop milking that favor" and "stop making me feel obligated" are two expressions I've heard so far. Though I'm not sure about their correctness. I was wondering if you could suggest an alternative. Thanks.
There is a useful word that avoids confrontation, but is still assertive, given by Lexico:
On even terms, especially because a debt or score has been settled.
It can be applied to favours too. Here are some ways to say it:
I think we're just about quits now, don't you?
Let's call it quits.
The second suggestion is better said just after a favour has been returned or repaid.