Does this post maybe help (It’s mentioning the same thing that @freumichCH has hinted at above - it’s in the sharing settings) Not sure if restricting sharing also has effects on general user visibility, but maybe worth a try?
I tried a quick search in the server bugtracker for an issue regarding this, but didn’t yet find anything; I’d recommend to do a thorough search there, maybe you find an according issue to which you could add your voice; if there’s nothing there yet, I would recommend you to submit a new issue there, so that the developers notice your need!
Edit: Also just found this other, older topic - there seems to be a way to disable autocomplete via config