You should use the word לא, just like you did. No need to use a different word.
As for the remark that Noam wrote - You can use the first לא and you can use the sentence without the first לא. It is just like in English. If I ask:
What is that? Is that bread?
You can answer it in both ways:
No, it isn't bread
it isn't bread.
Personally I'll answer the exact same that you answered - לא, זה לא לחם