No one has to proof anything. If A invites B and B invites C who acts openly bad, you can remove all parties at once and maybe revoke on appeal. All up to the community. Otherwise it would be indeed simple to defeat. But before banning A, one can also just give a Warning. No restrictions here in principle, but I am also open for concrete implementations that work well.
The point is that either there has to be a limit for how much you get punished for the acts of your grandchildren, which leaves room for motivated abusers to work around your system, or people can expect to be banned for basically no fault of their own if they ever invite anyone, in which case your system is DOA.
The point is, it is a balance each community has to find on their own. In reality this means adjusting depending on incidents. But if A invites B who openly does bad things, it very much is the fault of A to drag this person into the community.
