Since you and the girl you intend to marry are both adults, they can't stop you from doing so. However, the signatures, blank cheques can be used against you. In order to protect yourself, give a report to the local police station about the entire situation. Write down the details of the cheques and the papers you had signed. This might get you some sort of protection.
This apart, try and speak with your parents, reason with them that just by signing a few documents you can't be deterred. However if this does not work, you may also seek protection from the court.