How can I add a hidden admin?

I was wondering, if there was a way to add a hidden admin for a wordpress site that can’t be seen through the dashboard? Or any other killswitch?

I know, plenty of hackers have found a way already, since there are so many discussions about it out there ??

When I build custom websites I don’t upload them until I have been paid in full. Now, that doesn’t work, if you build the wordpress site live. For some themes/widgets/plugins with a lot of theme options, you can’t export all the options, so moving it is such an undertaking.

I’m currently out of a lot of money because my customer changed the password on me. I have a contract and it states that I reserve the right to remove any work until paid in full. I would really like to protect myself from non-paying customers.

Does anybody have a suggestion?

I guess, my other option would be to buy Backupbuddy and be able to move the whole site easier but it’s kind of expensive…

Are there any other plug-ins that help move the whole site including, plug-in settings, widget settings and theme settings?