Yes, that is one of the most common uses cases for sinon@2, but I believe that what you are looking for is neither a mock nor a spy, but instead a stub.

See: https://sinonjs.org/releases/v2.4.1/stubs/

var stub = sinon.stub(object, "foo");

//do your assertions

stub.restore(); //back to normal now.

This will allow you to create a default function that replaces object.foo().

From your question, however, it sounds like a no-op stub function is not what you are after, and instead want to override it with a custom function of your own:

var stub = sinon.stub(object, "foo", function customFoo() { /* ... */ });

//do your assertions

stub.restore(); //back to normal now.

HTH!

EDIT:

How to stub a single function:

The following stubs object.sourceAttackTarget()

var stub = sinon.stub(object, "sourceAttackTarget", function sourceAttackTargetCustom(gameState, source, target) {

    //do assertions on gameState, source, and target

    // ...

    return customReturn;

});

//do any further assertions

stub.restore(); //back to normal now.

How to stub a function to test for particular parameters

var stub = sinon.stub(object, "sourceAttackTarget");

stub.withArgs(1, "foo", "bar").returns("correctTarget");

stub.returns("wrongTarget");

var output = object.sourceAttackTarget(gameState, source, target);

equal(output, "correctTarget", 'sourceAttackTarget invoked with the right arguments');

//do any further assertions

stub.restore(); //back to normal now.

(Update)

There is also .calledWith() and .calledWithMatch(), which I just found out about. Could also prove quite useful for these sorts of tests.

How to mock just one method for an object:

"Mocks come with built-in expectations that may fail your test. Thus, they enforce implementation details. The rule of thumb is: if you wouldn’t add an assertion for some specific call, don’t mock it. Use a stub instead. In general you should never have more than one mock (possibly with several expectations) in a single test." - source

... so for the question asked above, a mock is not the right thing to use, and a stub is the appropriate choice. That being said, one of the scenarios can be tested easily with a mock, and that is the expectation that a method has been called with a particular set of arguments.

var mock = sinon.mock(object);

mock.expects("sourceAttackTarget").withArgs(1, "foo", "bar");

object.sourceAttackTarget(gameState, source, target);

mock.verify();