There is no easy way to do this. Most editors built with GTK+ use GtkTextView, but there is no control over glyph substitution in that module. Within GTK+, it is Pango that handles this. You can't change the behavior with a config file, but you can use the fallback attribute
to make a special build that doesn't take substitute glyphs.
At a lower level, font substitution for the whole system is managed by fontconfig, but there is no way to disable that feature in fontconfig.