Can't hide courtesy key signature if key changes to C major / A minor

Indeed, as there's nothing (no key sig) to select, and the corresponding inspector for the courtesy key sig doesn't allow for that setting (hint: this is where it should get fixed then)

Since this is a key change, there is an actual key signature to select, just nothing to click on. So instead, try Alt+Right starting from the clef.

Thank you! That was driving me nuts :). In a grand staff you have to do the same process for both treble and bass clef, then the courtesy natural signs disappear. Thanks!

Note, though, that hiding a courtesy key change to C major would normally be disastrous in performance - there is an almost 0% chance musicians would notice the change. If on the other hand this is an exercise sheet with each line in a different key, then no need to hide courtesy signatures individually, just turn them off altogether in Format / Style / Page. Or add section breaks at the end of each exercise, which has other benefits too.

And also, (2 yrs late, I know) you can choose format >style>page and not choose there to create courtesy key signatures. Same goes with time signatures and so on.

I write for example 1-2 row 4-8 bar melodies in different keys for teaching purposes and there courtesy key signatures are useless.

Use Section breaks then.

Indeed, section breaks are the much better solution there, because they also reset bar numbers, pauses playback, etc.