Actually, it hasn't been revealed. Considering that Shinigami make a big deal of Quincy being able do completely destroy souls, it's unlikely Hakuda and Kido would have that effect as well. And Shinigami had to use something to purify Hollows before Zanpakuto were invented. Then again, they did specify Zanpakuto are the only things capable of purifying Hollows... The only option that remains is that Hakuda and Kido neither purify or destroy a Hollow, but instead cause it to reïncarnate, just as when a regular Soul or Shinigami dies.