Hi Jon,
Sorry, NSubstitute does not support this.
As a fairly clumsy workaround you could strictly check calls after the fact using sub.ReceivedCalls(), which will return all the calls the substitute received. You could query this enumerable and make sure it only contains the calls you expect. For example, to check a substitute was not called at all you can do Assert.AreEqual(sub.ReceivedCalls().Count(), 0);.
Not exactly what you are after I know, but it's the closest I can think of, short of explicitly making other calls throw (from WhenForAnyArgs(...).Do(...) or ReturnsForAnyArgs(...)).
Regards,