What's alt+spacebar character and how to disable it?
optionspace inserts a non-breaking space (NBSP, U+00A0) by default.

What I've done I've set my editor highlight the non-breaking space, so that I see when I accidentally enter it.

You could remap ⌥␣ to a normal space by creating ~/Library/KeyBindings/ and saving a property list like this as DefaultKeyBinding.dict:

{
    "~ " = (insertText:, " ");
}