How to keep operator from remote-locking my modem?

Hi,

I'm getting service from a network operator that I don't trust. I want to prevent them from locking my modem. Should I lock the modem myself or is there a better way? Do I understand it correctly that if I lock the modem and keep the password to myselfthey wouldn't be able to unlock it remotely?

I couldn't find much information about rmtlock command, everyone just wants to unlock their modems. There's just 'rmtlock engage <netkey>' in modem's help, but no more description about the netkey. Is this the same as network id (id in net_enc segment of .opt file) or something else? If not the id, how can I find it? Is there a rational reason for network operator to want to keep it secret?