/silence *!*@*,~*!*@*.users.undernet.org gave me exactly what I was looking for. I was looking for a way to stop the attack against the shell while allowing our ops to be oped by the eggdrop.
Our eggdrop doesn't not reply to non-member. While using /ignore, I was asking the eggdrop not to display the attack but the shell was still attacked. Using /silence the attack stop at the Undernet server, nothing reaches the shell. At least, this is my understanding.