What are the issues with Web Service in a web application?

"I mean web service can be used unnecessary or hacked for bandwidth wastage."

These are not typically web service issues these are issues for any web application whether its a web service or not a web service but web application.

Web Services are one of the principles that helps you design scalable and most  usable web applications. 

Bandwidth

Solution: 

1. Reduce the bandwidth usage by means of caching.

2. Serve less payload for instance JSON is better than XML in terms of size.

3. Later on charge consumer for consuming your services :)

Hacking/Security

Solution:

1. Regarding Hack you can implement security by authentication first and then authorization and there are many other ways for implementing security.

Just think of Amazon Services, Google Web-services, and other web-services that you daily consume in you web apps.

Please Elaborate your question.