If you're using NS 6  Under the Configuration tab
Go to: Server Settings > Notification Server Infrastructure > Microsoft Active Directory Import
There is a specific line for Imporitng User types.  Go through those settings to verify they're all set properly and that it's enabled.

If you're extending that into your helpdesk module then Current AD users will get updated properly.

However, removing is a much more difficult process.  Part of the reason is that many of them will already have incidents assigned to them and deleting that User object will cause issues with those tickets.  Typically they're left in Helpdesk module to preserve that historical data.