Removing accidentals from a single voice

The accidentals engravers engraver usually live at Staff level, but reads the settings for Accidental at Voice level, so you can \override them at Voice. To get rid of all accidentals, you need to remove both the Accidental engraver and the AccidentalCautionary one.

\version "2.24"

%% http://lsr.di.unimi.it/LSR/Item?id=758

SampleMusic = { cis cis cis! cis? }

LongMusic = {
  \SampleMusic

  \override Voice.Accidental.stencil = ##f
  \SampleMusic
  
  \override Voice.AccidentalCautionary.stencil = ##f
  \SampleMusic
  
  \revert Voice.Accidental.stencil
  \revert Voice.AccidentalCautionary.stencil
  \SampleMusic
}

<<
  \relative c'' {
    \LongMusic
  }
\\
  \relative c' {
    \SampleMusic
    \SampleMusic
    \SampleMusic
    \SampleMusic
  }
>>