I was looking for the same thing. I ended up adding the following extension methods to implement ShouldBeNull and the even shorter ShouldNotMap:
In RouteTestingExtensions.cs:
    /// <summary>
    /// Verifies that no corresponding route is defined.
    /// </summary>
    /// <param name="relativeUrl"></param>
    public static void ShouldNotMap(this string relativeUrl)
    {
        RouteData routeData = relativeUrl.Route();
        routeData.ShouldBeNull(string.Format("URL '{0}' shouldn't map.", relativeUrl));
    }
    /// <summary>
    /// Verifies that the <see cref="RouteData">routeData</see> is null.
    /// </summary>
    public static void ShouldNotMap(this RouteData routeData)
    {
        routeData.ShouldBeNull("URL should not map.");
    }
In GeneralTestExtensions.cs :
    ///<summary>
    /// Asserts that the object should be null.
    ///</summary>
    ///<param name="actual"></param>
    ///<param name="message"></param>
    ///<exception cref="AssertFailedException"></exception>
    public static void ShouldBeNull(this object actual, string message)
    {
        if (actual != null)
        {
            throw new AssertFailedException(message);
        }
    }