A better access protection?

Is there a way to protect the struct Info in the class? I want to make it a 'sealed class' but that is illegal.

    public class Guestbook_Lib : Basic_Valid
    {
        public struct Book//Main Book Structure
        {
            static string[] Array;
            static Info[] i;
        }
        struct Info //For security, Protect Contact information
        {
            private int Team;
            private string name;
            private string contact;
            private string location;
            //private void Send(Listbox l)
            //{

            //}
        }