to really want or not to really want

I noticed that in the grammar section of "No, thank you" that it warned against using "hen3, zhen1, fei1 chang2" before the verb "yao4". But, in the dialogue the woman says, "zhen1 de bu4(2) yao4." So, I see that it is acceptable in declaring that you don't really want something, but unacceptable when you do really want soemthing. Why? I hope to get answer to this. The following question would then be, how do say that you really want something?