aweber api

In Aweber I have 2 lists. A. For prospects B. For members who bought my digital course. When the prospect buys the course, they subscribe to the members list and automatically unsubscribe from the prospect list. Perfect! Now I want to prevent members to be able to subscribe AGAIN to my prospect list. It should show them a message like: "You already are a member, you can watch all the free videos in the course you already purchased" Who can help me with this? Thank you! Matthias