Hello @nec! Welcome to the Collabora Online forums! 
It’s quite interesting to hear that you want to restrict document editing to a single user, especially considering that Collabora is designed for multi-user simultaneous editing! 
@nec, could you please share the specific reason for this requirement? We’ve had a similar inquiry before—feel free to check out this link: Is there any possibility to decrease the number of simultaneously opened documents?
Cheers,
Darshan