How to authenticate method on a web service so that hackers could not invoke methods within webservice using different tools?

We have developed web service(as well as generic handler)  based web application.But it seems that  the methods could be invoked by using different tools like (Poster,Advanced Rest Client) to get the secure information of Administrator and could do malicious activities.We would be grateful if you provide us the best possible method to make the method inside webservice or generic handler becomes secure.