With current size of the network, it's probably managed by sending messages to operators chat “Hey, IP a.b.c.d is doing that again”.

Remember that Fido and Usenet relied on independent server admins voluntary enforcing the rules for global groups (and allowed the alternative sister hierarchies or local appendices with different rules). It is possible to give more power to local decision maker, and share the global ideas.

Link establishment mentions validation of the circle by the intermediate hops. I suppose that someone who is sending a lot of packets without participation from the other side can be put into exponentially worse and worse queues. Or maybe not. There's a lot of things to test.