How do you reply to IL n'y A pas de quoi?

"Il n'y a pas de quoi" means "you're welcome / don't mention it". As this is already someone replying to a 'thank you very much', it does not seem that you should reply again. If you are thinking that you should because the person did you a very great favour, you may reply in turn "mais si, je ne sais pas comment j'aurais fait sans votre aide" (I don't know how I would have done without your help) or any other formula that you like, as there is no traditional one.