DreadMoth: Does anyone know of a way to (permanently) erase spells from the spellbook? I don't mean memorised spells, but ones that are copied from scrolls/artifacts into the spellbook.
Sorry, you can't do it.
Once copied in the spellbook, a spell stays there for ever.
If you want more spell space available in one's spellbook (when applicable) then you should level up that specific character (in case of TNO you should raise intelligence when leveling up)
Don't worry for your little error, you should easily find scrolls of Magic Missiles either through buying or looting (it's not that good as a spell).
Hope that helps.