Since the multiboxers are obviously using a botting program to autoaccept the invite to wintergrasp (how they get all their toons in quickly) that's reason enough to ban them.
but yet as dilong said, just prevent multiboxers from joining wintergrasp and bgs.
the other option is to increase the effect of tenacity in that a single boomkin running into a multiboxer hitting starfall would decimate the multiboxer.