How can I delete users from the NAB using a LotusScript agent or actions on a database?
Use a Notes 6 server and client. If you're going to be doing a lot of this stuff it makes sense to upgrade at least one machine just so that you can write programs to automate it. Use the NotesAdministrationProcess class, DeleteUser method.
In Notes 5, you can use the NotesDocument.Remove method in the Domino Directory, but that doesn't clean up user mail files, add them to deny lists, remove them from other groups and so on. What you really need to do is not just remove the users, but activate the adminp functions to clean up after them.
In R5, you could perhaps write (or copy from somewhere else) code to create adminp request documents, but I don't have such code handy myself. The best way to see what fields are in these requests is to experiment: turn off adminp, use the administrator client to create a request, and then go into the administration requests database on the server and see what fields are in it.