Have you tried MyEditor.SyntaxHighlighting = null
?
This works for me:
<Grid>
<Grid.RowDefinitions>
<RowDefinition Height="*"/>
<RowDefinition Height="Auto"/>
</Grid.RowDefinitions>
<avalonEdit:TextEditor SyntaxHighlighting="C#" x:Name="TextEditor">
public class Foo
{
}
</avalonEdit:TextEditor>
<Button Grid.Row="1" Content="Reset syntax highlighting" Click="Button_Click" />
</Grid>
Code-behind:
private void Button_Click(object sender, RoutedEventArgs e)
{
TextEditor.SyntaxHighlighting = null; // highlighting disappears
}