Emeralds by Night
There are three means by which magick can be countered.
Countermagick is using one Sphere to directly counter the same Sphere. A student of Forces can shut down another mage's attempt to throw a fireball, or a student of Spirit can stop another mage from summoning a demon. This requires an Arete roll of difficulty 8, and each success negates one success of the original Mage's working.
Anti-magick uses Prime to literally nullify magick of any Sphere: its effects make things look clearer and less fantastic, shrinking the Demimonde and instilling disbelief in the observer. This requires an Arete roll of difficulty 8, each success increasing the original's mage's difficulty by 1. So for example, a Matter 2 effect of turning straw into gold would be difficulty 6, or 7 with witnesses. 3 anti-magick successes would increase that difficulty to 9 (or 10), changing how successful the original spell was. Other Initiates of Prime can buffer or guard against this effect.
Finally, a free-standing spell such as a ward or illusion can be unraveled or unwoven. This requires knowledge of Prime and at least some of the Spheres used, however, it is much easier as it isn't urgent, no more difficult than the difficulty of the magick itself (or difficulty 8, whichever is easier).