Which part of the word "no" you didn't understand
?What part of no don't you understand
? And then you explain why the "no" (that was even put to use in a song) What part of no don't you understand?
To put it plain and simple
I'm not into one night stands
I'll be glad to explain it
If it's too hard to comprehend
So tell me what part of no
Don't you understand?http://www.phrases.org.uk/meanings/what-part-of-no.html
In more informal/colloquial way: "what part of no you don´t understand?" Be aware that the other party could answer "the honest answer is that you don't understand the bit that's not yes." ha ha ha!
But back to the crux. You could state "what part of no you don´t understand, I am not going to give you a second chance." or even "with me there´s no second chances, what part of "no" you don´t understand?"