[Mono-list] Final method in C# (as the final keyword in Java)

[Ympostor]

Simple question (forgive my ignorance):

     public abstract class GrandFather
     {
         public abstract void DontOverrideMe();
     }

     public class Father : GrandFather
     {
         public sealed override
             void DontOverrideMe()
         {
             Console.WriteLine("I am acting as a father");
         }
     }

     public class Son : Father
     {
         public void DontOverrideMe()
         {
             Console.WriteLine("I am an unruly son");
         }
     }

Why the compiler only gives a warning about Son::DontOverrideMe instead 
of an error? And why with a "new" keyword the warning disappears? I want 
a non overridable method :(

Regards.

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