Detect life.
A bow enchanted with 1 or 2 seconds of any of the following: Drain speed 100, Demoralize, Command Creature.
Yet, proper use of those two things can tear up a wisp safely, even at close range.
I imagine wisps represent even less of a challenge to a reasonably prepared mage.
As far as a melee character, one would think that turnabout is fair play - ie: a blade enchanted with absorb life.
I suspect it is more a matter of player experience. New players who stumble upon wisps are likely to be unpleasantly surprised until they visit these forums.