That doesn't solve anything. How far up the invite tree do you go? If you go all the way to the top, you'd have to ban yourself, the founding user. If you don't go all the way, then unsavoury users will just construct a long invite chain to insulate themselves from whoever they invite. There's probably some more clever graph theory stuff you can do to make this process a bit more reliable, but good luck justifying your algorithm to the people you just banned.