I sent them signed message from the staked address you quoted but they're not ready to accept it.
They think that my account is still compromised and they are not sure whether it would be accessed by me or someone else once they unblock it?
What other options can you guys think of to convince them that it's actually me only who will access the account once it's unblocked?
Unfortunately signing the message is the closest you can get to recover your account. So far I have not seen any other options such as providing login hour and time and device model unless someone could prove otherwise.