hitInfo hit;
float nullify = 0;
hit.damageTypes.Scale(DamageType.Explosion, nullify);
This is my method but I get an error says "Scale takes 3 arguements"

I'm not sure how that's possible since Scale only has two arguments. Are you seeing that error in your editor and/or are you seeing that when Oxide tries to compile the plugin?

Occurs during compile, I'm up to any alternative way to stop explosion damage.

Merged post

was missing hit.damageTypes = new DamageTypeList();
but didn't solve my situation I just used
hitInfo hit;
hit.damageTypes = new DamageTypeList();
hit.DidHit = false;
hit.HitEntity = null;
hit.Initiator = null;
hit.DoHitEffects = false;
hit.HitMaterial = 0;
hit.PointStart = Vector3.zero;
hit.HitBone = 0;

One thing is it's HitInfo not hitInfo. I'm not sure what the lowercase one would even point to. I also don't know why you would be declaring a new one anyway, since the hook passes one in.