I think I know where this is headed...

HEAD KNIGHT: Augh! Ohh! Don't say that word.
ARTHUR: What word?
HEAD KNIGHT: I cannot tell, suffice to say is one of the words the Knights of Ni cannot hear.
ARTHUR: How can we not say the word if you don't tell us what it is?
KNIGHTS OF NI: Aaaaugh!
HEAD KNIGHT: You said it again!
ARTHUR: What, 'is'?
KNIGHTS OF NI: Agh! No, not 'is'.
HEAD KNIGHT: No, not 'is'. You wouldn't get very far in life not saying 'is'.
KNIGHTS OF NI: No, not 'is'. Not 'is'.

And so on. The answer is to make language impermissible for them.