> I send a message to all logged on users to close down so I can back up.

Once done I

> want to lock the database so no one can do anything while I back it up.
>
> How can the locking (exclusive use) be done?

Do a database shutdown. Then, restart it when done.

But, if you are only doing a backup, you don't actually need to lock down
the database.

HTH,
Jason Wharton
CPS - Mesa AZ
http://www.ibobjects.com