To politely tell someone to stop asking if you're busy, you can use a courteous and direct approach. Here's an example of what you can say:
"Thank you for checking in on me, but I'd prefer not to keep discussing my schedule. If I have some free time, I'll reach out to you. Let's focus on our conversation/another topic for now."
This response acknowledges their concern while firmly setting a boundary. Remember to maintain a polite and respectful tone in your communication to avoid any misunderstandings or hurt feelings.