I'd suggest checking out the API Examples for Email Marketing and Newsletter Recipient as well as the API Documentation for the ISubscriptionService interface.
What you'll end up doing is:
- Get ALL of your contacts (with valid email addresses and any other where parameters you wish),
- Get ALL of your email feeds,
- Loop through all your contacts,
- Inside that loop, check to see if the contact is subscribed to the newsletters in your list
- If they are not, add them to a list for "deletion" or delete them at that time.
Give this a shot and see what you come up with.