How to omit zero value line in contourm figure

Hi I want to have a contour plot without zero value line,

I have been search internet and matlab Q&A for a long time,

but can't find answer , how can I do this ?

thank you

this is my figure link

https://docs.google.com/file/d/0BxUY6-ywKIvpUjF5VlIwZFU0VkU/edit?usp=sharing