I like the Destroy the Spellbooks option. How much would it cost to get a group of five high-level rogues to assist your army? Cast a permanent Detect Magic on each of them, give them some powerful magical items, and send them in to destroy spellbooks in a coordinated assault: all spellbooks should be destroyed within two minutes of one another.
The main problems with this plan are the first-level spell Alarm (which is pretty difficult to get around) and discovering the spellbooks in the first place. Several castings of Ethereal Jaunt and Locate Object could solve these problems, but then you're getting into some very high-level magic. Alternately, if the names of the wizards are known, Locate Object scrolls, combined with rapid force attacks, could work pretty well. Zip in, get the spellbooks, and destroy them.
Note that a thin sheet of lead blocks the spell. How many wizards have you ever seen in any game who keep their spellbooks behind a thin sheet of lead?
Daniel