Necro...
Here was my simple way of dealing with spell spam. Basic system was the same.
Every spell cost its level: 1st cost 1, 3rd cost 3, etc...
The Mana pool was taken from what the character could cast. So, if a first level player with all bonuses could cast 4 first level spells, the mana pool was 4 pts. The character still had to study spells in the morning. No more than the usual slots. However, they could then use the slots in any of the spells they studied.
As for the spam. Each time a character cast a spell he did not specifically use a slot for, the cost was doubled.
Example:
Raven is a 1st level mage that has 4 spell slots. He knows he is going into combat, so he really concentrates on his magic missle(2 slots) and then rounds things out with burning hands and sleep. In the course of the day he can cast any of those normally. However, if he chose to cast a second burning hands, sleep, or a third MM, the cost would be doubled.
I know it is a steep cost, but I believe giving the wizard this option increases his power dramatically. Plus, I also have a house rule for pushing. So, the wizard could squeeze a bit more out if in serious need.