Oh you too! My ban came from discovering where the station number (the 8 bit number uniquely identifying each BBC Micro on the Econet network - like a MAC address) was stored in memory. I did this by the obvious technique of searching memory for this number on several machines and comparing the addresses - it was address 3361. Then I found that by changing this you could impersonate the credentials of the person logged in to the other machine, although it worked better if you also had a co-conspirator turn off their machine at the same time.
The other hack which possibly contributed was writing a kind of "broadcast NOTIFY" that sent your keystrokes to all of the other machines on the network, allowing the user to remote control all the things. The service manual I just found online says that *NOTIFY is a dangerous command which the operator might want to remove - but in fact there was a system call which did the same thing and I used that instead.