CANCELING A SPELL
The caster of a spell can, at any time, cancel that spell; the caster must be within the spell’s original range limit or within “sensing” range of the spell’s current effects. The caster cannot cancel the actual physical or mental results of a spell (e.g., fire, wounds, created materials, etc.). Cancellation takes 1 round of concentration. (Channeling spells may also require the permission or acquiescence of the caster’s deity to cancel the effect.)
Spell Law pg 87