Securing 3rd party webservice



I would like some advice please.

We are building a flex application which is consuming a 3rd
party web-service. We want to make sure that only the flex
application we made can have access to this 3rd party web-service.
The flex application will be publicly available, so there is no way
of putting in some authentication here. How should I configure the
proxy-config.xml to prevent other consumers (other than our flex
app) from using the web-service available through the flex proxy.