I have been buying the moneypaks with credit cards at various stores for the past two months. I got shut down this week with a similar outcome as the OP (I can use up the card and cannot knock on their doorstep again).

I had a few and never used them for everyday purchases (mistake #1?). I loaded them and drained via the bill pay system, only (mistake #2?).

Green dot refused to tell me why they shut me down. I could have pursued it with the supervisor but the customer service rep placed me on a long hold and never came back.