IS IT possible?
The Knights happen to have a weakness in that a number of words, when spoken to them, has the same effect on them as their saying "Ni" has on others. The only one of these words that is revealed in the film is the word "it", which is picked up on when Arthur, upon being asked to cut down the mightiest tree in the forest with a herring, declares that "it can't be done." Ultimately, King Arthur and Sir Robin (who shows up with his own knights) both say "it" several times in conversation (unaware that "it" was the word that was causing the Knights' pain). However earlier in the very same scene the head Knight himself uses the word ("It is a good shrubbery.") suggesting that it only causes the knights pain when said by someone else.