What should I do, as I dealt with a bank through usury unknowingly, and I refused to repay the usurious loan after learning of its prohibition, and the bank is threatening me with imprisonment?

If you predominantly believe that those in charge of the bank will carry out their threats to take the matter to a court that does not abide by Allah's Sharia, and you fear unjust rulings such as imprisonment or seizure of property, then there is no blame upon you for paying the usurious loan interest to be safe from legal prosecution. You must clarify to those in charge of the bank and the court that your refusal to pay this interest stems from a religious conviction and compliance with the Almighty's saying: (But if you do not, then be informed of a war [against you] from Allah and His Messenger. But if you repent, you may have your principal – [thus] you do no wrong, nor are you wronged). You must repent, seek forgiveness, and resolve not to return to such an act.