Brain Fog

You target the brain of a creature you can see within range, preventing it from focusing on spells. For the duration of this power, the creature cannot cast spells of 1st-level or lower unless it succeeds on an Intelligence saving throw. If the creature is actively concentrating on a spell, it must succeed on a concentration saving throw against your spell save DC to keep concentrating on that spell. 

At higher levels, whenever the target attempts to cast a spell with a level equal to the level of casting, up to and including level 5, it must first succeed on a spellcasting ability saving throw or lose its concentration on the spell.