Nope! It's not polite.

Not on your Nelly!

I think "nope" is just fine, it's just more casual. "No" is a little more formal.

We don't use it I think they use it in USA. Just say no is best

The "p" sound in "nope" gives it a sense of finality (same with "yep"). In a formal context, this level of bluntness (and slanginess) is not acceptable.