How to hide (remove) a base class's methods in C#?

The essence of the problem is, given a class hierarchy like this:

class A
{
    protected void MethodToExpose()
    {}

    protected void MethodToHide(object param)
    {}
}

class B : A
{
    new private void MethodToHide(object param)
    {}

    protected void NewMethodInB()
    {}
}

class C : B
{
    public void DoSomething()
    {
        base.MethodToHide("the parameter"); // This still calls A.MethodToHide()
        base.MethodToExpose(); // This calls A.MethodToExpose(), but that's ok
        base.NewMethodInB();
    }
}

How can I prevent any classes that inherit from class "B" from seeing the method A.MethodToHide()? In C++, this was easy enough by using a declaration such as class B : private A, but this syntax is not valid in C#.

For those interested (or wondering what I'm really trying to do), what we're trying to do is create a wrapper for for Rhino.Commons.NHRepository that hides the methods we don't want to expose to our group of developers, so we can have a cookie-cutter way of developing our app that new developers can easily follow. So yes, I believe the "Is-A" test is valid for the whole chain (WidgetRepository Is-A BaseRepository Is-A NHRepository).

Edit: I should have mentioned, for the sake of argument, that class A is an API class outside of our control. Otherwise the problem gets considerably easier.