Dispel:

If successful, contesting magic checks are made between the two affected characters. If successful once again, the target spell fails. If either character exceeds the other by 5 or more, the character who failed suffers a Magical Malfunction. If the Initial Cost of Dispel is less than half the Initial Cost of the spell it is countering, rounded up, increase it to that amount. Dispel may also be cast on spells that already exist, and need to be maintained. Dispel may only be used to prevent the casting of a spell that does not already exist once. More can be cast after the effects of the original spell are triggered, if those effects last.