Active Directory Inactive User Detection

Hi all, I know that I could use System.DirectoryServices to gain access to Active Directory using c#. Now here is my question, do anybody know which method I could use to detect inactive user accounts? If you are aware of, every time the user creating a new user accounts, there will be a account varification which will sent you an Email to redirect you to the website to complete the varification. My intension is to delete those user accounts that created but did not complete the varification part, and also the user who never log in within the pass 30days. Thanks for any input, will be helpful if you guy can give some comments. :)

Code:

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using System;
using System.Collections.Generic;
using System.Text;
using System.DirectoryServices;


namespace RemoveInactiveADUser
{
    public class DeleteObject
    {
        public static void Main()
        {
            DirectoryEntry de = new DirectoryEntry(
              "LDAP://192.168.1.100/ou=accounting, dc=ispnet1, dc=net",
              "cn=Administrator, dc=ispnet1, dc=net", "password",
              AuthenticationTypes.ServerBind);

            DirectoryEntries children = de.Children;
            try
            {
                DirectoryEntry badObject = children.Find("ou=auditing");
                badObject.DeleteTree();
                de.CommitChanges();
                Console.WriteLine("the object has been deleted");
            }
            catch (Exception e)
            {
                Console.WriteLine("the object was not found or deleted:");
                Console.WriteLine(e.ToString());
            }
        }
    }

}

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is it very hard to achive that?