How far up the tree do you kick? Going too far up makes it so malicious people can "sabotage" by botting to get huge swatch of legitimate users banned.
Going to shallow means I just need to create N+1 distance between myself and my bot accounts
Your tree could for instance be pruned - you can still invite people, but the people you invited can no longer invite people.
There are not a lot of sites which have tried this and failed. Those which have tried to be even a little bit clever about it, have succeeded pretty well (Advogato was a really early example).
What there have been, are sites which rejected such restrictions after a while, because they would rather have a big number to show to investors than real people. Many have even run the fake accounts themselves (e.g. Reddit).
