When an object is deleted from Active Directory, it is not actually removed from the database, as this would hinder replication of the deletion. Instead, the object is tombstoned. This tombstone prevents the object from being usable (for logins, for example) and being visible in all, the common Active Directory tools. It also instructs the garbage collection process on each domain controller to remove the object from the database, once the tombstone lifetime period has expired.
To modify the tombstone lifetime period, you'll need to be signed in with an account that is a member of the Domain Admins group.