Van de Graaf is positive charged, balloon not charged, head is negatively charged.
Ball bounces to VdG, becomes positive by throwing of all electrons, repels from the VdG and head to the head.
ballon reaches head and fills up all the missing electrons, however it gets more electrons than necessary, because the electrons in the human can push each other "into" the ballon, so the Balloon sooner or later is repelled by the head.
The question is: Why is the human negatively charged or constantly negatively charged, just like the VdG is positively charged.
I can imagine that the VdG is always positively charged by magic, dunno, how it works, I don't know, why this should also apply to the human.
The only explanation I have would be, that the earth is strongly negatively charged and just "begs" for more space to charge something negatively.