If you're looking for the opposite of a positive force, you're misinterpreting what exactly a Force is.
A Force is what's known as a vector: it is a value that has both a quantity and a direction. In most cases, the quantity is Newtons (a unit of force) and ANY prescribed direction (right, left, down, up, etc.)
A Force is positive if it acts toward the prescribed direction and is negative if acts in the opposite direction.
Consider the following:
If we consider "Up" as positive Force, then the pull of the rope is a positive force acting on the climber. Therefore, the pull of gravity is a negative force acting on the climber.
The idea of a "negative" force is about perspective.
Now let's assume you mean force in an abstract way:
Lightning is actually not a force itself, but the rapid movement of electrons to a electron deficient space. Simply put, it moves so fast that it releases immense amounts of energy, causing the shockwaves we call "thunder".
There are a couple ways you could conceive of an opposite of lightning.
- Lightning, but in the opposite direction would have a similar Physical Effect, but on the receiving end of the bolt. That may not be ideal for stereotypical damage, but since its magic, perhaps you just drained all the electrons from the victim suddenly? That could be fatal with a little hand waving.
- Protons, the opposite charged particles of electrons, could instead be drawn to your weapon. Since Protons are fundamental particles in atoms: you get a flash of lightning, and BOOM. The person has vanished in atomic particles. In Physics, this is basically impossible, due to some more complex forces, but since you're using a magic system, it is at least rooted in duality.