can't remove gratuitous sharps/naturals

It’s to clarify that you really do mean a D# in one octave and a Dnatural simultaneously in another octave. Select the relevant notes (it’s fine if you select the whole chord) and flick the Accidental switch in the Notes and Rests section of the properties panel. Then, if it’s not already selected, choose “Hide” from the dropdown.

(also documented here)