The thing with PKing is that there's not necessarily any strict power limit for it.
Saying that someone is a PKer is like saying that someone is a magus - It basically explains the source of his/her power, but it doesn't speak to the extent of it.
The most elite PKers in this world might wield a degree of power on par with Magneto (since we already have one X-Men reference on this thread
). Once superpowers reach
that level, there really isn't anything Earth governments can do about it, short of dropping nukes, perhaps...