how we stop a class being inherited in .Net.

by putting "Sealed" keyword, you can achieve your target. sealed keyword do not let any class to be inherited by other class.
I would also suggest you to please first learn access specifiers and then do start programming other wise, you structure will begun dangling at some place.

You can create it as Sealed class. Sealed class cannot be inherited.
Ex.
public sealed class yourClass {..}

Make it a sealed class when writing the class. sealed a keyword used when you want to just initialize the class and use the object and do not want any other class to inherit it.

For Examples:

public sealed MyClass
{
    //Class members
}