Did you chance the security code from the default 12345? I don't know of any other way to hard reset it except by flashing it with new firmware (if available). Nokia didn't make self-service ROMs available for home flashing for older phones though, so you'd need a service center to do this. There are online sites (mostly Russian) that offer to generate a master code for your phone based on its IMEI code but I don't know how well they work or if they're reputable.