It should be interpreted that you are unable to gain the Spell in the first place as they are not treated like Objects.
The rules state on page 13 - " All characters may acquire and cast Spells, if their Craft is sufficient to permit this. "
So if you Craft is not sufficiently high, or you are already carrying your full complement of Spells, you may not acquire another.
" Any character acquiring more than four Objects must decide which to keep. The remainder are immediately placed faceup on the character’s space. "
This applies to when you find or take an Object and make a choice as to which to carry with you.
" If at any time a character has more Spells than his Craft allows, surplus Spells must immediately be placed on the Spell Card discard pile; they cannot be cast. "
This only applies to if your Craft were to drop for some reason.
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