Aha! I just had this problem this past summer, but it was just the normal squirrels. My husband had to rent a trap (won't harm the squirrels)and put a lump of peanut butter in the cage to attract them (they love it!) Sounds like you have a family of them, so it's going to take you several trips. Took my hubby 7 trips! We had a good laugh about it.
Be sure that you note where you dropped the flying squirrels off (drive about 5 - 10 miles) find a wooded area and let it loose. Then be sure you bring the rest of the family of squirrels to the same spot. If not, they'll hunt for each other and you could well have them back.
If you have been feeding them ... please stop. Even leaving dog food or any food for pets outdoors is an attractions for squirrels.
Good luck Happy New Year Marcy