Is there a option or plugin or similar tho "convert" ZenPhoto into a only single user CMS? So, in which all multi-user functions are turned off. Also because of more security.

You can use Zenphoto with just one user without any issue.

What specifially do you wish to turn off else? Things like self registering or user groups are just plugins.

OK. I want to prevent anyone else from registering or logging in.

Okay, then that should have you covered.