Dragonbane

This spell prevents any dragon who fails its saving throw from entering the area of effect. The spell affects a cubic area whose sides equal the caster's level times 5 feet; thus, a 16th-level caster could affect a cube whose sides each equal 80 feet. The dragon can cast spells, blast breath weapon, or hurl missiles (if possible) into the area of effect.
Dragons within the area of effect when the spell is cast are not affected. If such dragons leave the area of effect, they must succeed a saving throw to reenter the area.
The spell's effectiveness can be greatly increased with the casting of a Focus spell.

[Tome of Magic]

Sphere: Wards
(Abjuration)
Level: 6

Range: 10 yards/level
Components: V, S, M
Duration: 1d4 rounds +1 round/2 levels
Casting Time: 1 round
Area of Effect: 5'-cube/level
Saving Throw: Negates

Material Components:
the priest's holy symbol and a dragon scale