For reference types (strings, objects, etc), just make the value null -
null references aren't serialized.
For value types, you have to add a "friend" to your code... eg

public int myFoo;
public bool myFooSpecified;

To supress myFoo (which always has a value, which is by default false)
serialization, set myFooSpecified to false (the default).  To make it get
serialized, set myFooSpecified to true.

No matter what your value type field (or property) is, a boolean friend
with the same first name and Specified at the end will do the trick.

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