Mobile Legends Diamonds Refunded, Now Account in Negative Balance

My younger sibling accidentally purchased diamonds in Mobile Legends, and after processing a refund, my diamond balance turned negative, and several of my characters were banned. Despite my efforts, my sibling continues to find ways to purchase diamonds, which I can’t seem to prevent. When I logged in recently, I could only play the free characters. I need help resolving this issue and restoring my account. Please assist as soon as possible.