unbinding mouse buttons

I'm trying to unbind the left-hand mouse button, but can't seem to get it to un-function! From my AIS_ViewController descendant class...

    myMouseGestureMap.Clear();

    myMouseGestureMap.UnBind(Aspect_VKeyMouse_LeftButton);
    myMouseGestureMap.UnBind(Aspect_VKeyMouse_RightButton);
    myMouseGestureMap.Bind(Aspect_VKeyMouse_MiddleButton, AIS_MouseGesture_RotateOrbit);

running up the code still has the left button performing rotation (looks like AIS_MouseGesture_RotateView)

Also tried...

    myMouseGestureMap.Bind(Aspect_VKeyMouse_LeftButton, AIS_MouseGesture_NONE);

but doesn't make any difference.

This is fed into the mouse handler:

void occView::mousePressEvent(QMouseEvent* event)
{
    Graphic3d_Vec2i point;
    point.SetValues(event->position().x(), event->position().y());
    const Aspect_VKeyFlags flags = qtMouseModifiers2VKeys (event->modifiers());
    if (!_view.IsNull() && UpdateMouseButtons(point, qtMouseButtons2VKeys (event->buttons()), flags, false))
        updateView();

    _clickPos = point;
}

Can someone check my sanity here, or does this look like a bug?