If you don't want to allow all the methods to be called, simply don't publish them. From the calling side point of view the client sees only the methods your interface defines. You can decide what to include in the interface and what is not inlcuded.
If you want to include all of the methods in the interface in the beginning, you can prevent further execution in your code, just return from the method.
There are quite good tutorials about creating WCF services, for example
Implementing a Basic Hello World WCF Service (v4.5)[
^] or
WCF: From a Beginner's perspective & a Tutorial[
^]
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