You're not going to believe this but it's already there, just commented out!

In MageAI.cs find:

Code:
        public virtual Spell GetRandomFieldSpell()
        {
            // I left this here if someone wants field spells
            /*bool pois = m_Mobile.Skills[SkillName.Poisoning].Value >= 80.0 || m_Mobile.HitPoison == Poison.Greater || m_Mobile.HitPoison == Poison.Lethal;

            if (pois && CheckCanCastMagery(5))
                return new PoisonFieldSpell(m_Mobile, null);

            else if (Utility.RandomBool() && CheckCanCastMagery(6))
                return new ParalyzeFieldSpell(m_Mobile, null);
            else if (CheckCanCastMagery(4))
                return new FireFieldSpell(m_Mobile, null);*/

            return null;
        }

Now that you've mentioned this I think I'll un-comment it on my server too.
 
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