A spontaneous caster can not even access the Spell Pool. To cast a spell from the spell pool, requires an:
"open, unused spell slot of the appropriate level. Wizards preparing spells for day decide at the time wether to leave some spells open."
Spontaneous casters do not have open spell slots, spontaneous casters only have used and unused spell slots.
Here is the text from the SRD re: Wizardly spell prep:
When preparing spells for the day, a wizard can leave some of these spell slots open. Later during that day, she can repeat the preparation process as often as she likes, time and circumstances permitting. During these extra sessions of preparation, the wizard can fill these unused spell slots. She cannot, however, abandon a previously prepared spell to replace it with another one or fill a slot that is empty because she has cast a spell in the meantime. That sort of preparation requires a mind fresh from rest. Like the first session of the day, this preparation takes at least 15 minutes, and it takes longer if the wizard prepares more than one-quarter of her spells.
Here is the text from the SRD concerning Spontaneous Casters:
[I]Daily Readying of Spells: Each day, sorcerers and bards must focus their minds on the task of casting their spells. A sorcerer or bard needs 8 hours of rest (just like a wizard), after which he spends 15 minutes concentrating. (A bard must sing, recite, or play an instrument of some kind while concentrating.) During this period, the sorcerer or bard readies his mind to cast his daily allotment of spells. Without such a period to refresh himself, the character does not regain the spell slots he used up the day before.[/I]
No mention of Open spell slots
Leaving slots "Open" is the solely a power that Wizards have,(and other Preparers), Moreover, while Arcane Preparation could let a Warmage get into Mage of the Arcane Order, the feat description clearly states that spell slot so used can only be used to cast the prepared spell. Of course under Leaving Slots Open, above,( which a Warmage does not get access to anyway), it specifically states; [B]" She cannot, however, abandon a previously prepared spell to replace it with another one[/B] ".