There's no way to get that granularity. Automatic signatures can be either on or off, not in between.
The details: Signature files for Outlook 2003 reside in the %appdata%\Microsoft\Signatures folder. There are three ways to make Outlook to use a particular signature with all accounts:
1) Apply Office Service Pack 2 (to resolve the problem discussed in MSKB article 898076) and then set the NewSignature and ReplySignature string values in the HKEY_CURRENT_USER\Software\Policies\Microsoft\Office\11.0\Common\MailSettings key to the name of the signature. This applies the signature as a policy setting, so the user won't be able to change the automatic signature (and may also be blocked from some other features on the Tools | Options | Mail Format dialog). If the signature files in the Signatures folder are blank, then the automatic signature inserted will be just blank text.
2) Same as #1, only set the NewSignature and ReplySignature string values in the HKEY_CURRENT_USER\Software\Microsoft\Office\11.0\Common\MailSettings key. This applies the setting as a user preference.
3) Use a script like that at
http://www.outlookcode.com/codedetail.aspx?id=821 to apply the signature to each account. This is the brute force method, available if you can't apply SP2 for some reason.
Note that even if you mandate a signature with a Group Policy Object, the user can still delete the signature from the message itself, and the savvy user can edit the .rtf, .htm, and .txt files for that signature. That's why if you really want a mandatory signature, you must do it on the server; see
http://www.slipstick.com/addins/content_management.htm for links to tools.
--
Sue Mosher, Outlook MVP
Author of Configuring Microsoft Outlook 2003
and Microsoft Outlook Programming - Jumpstart for
Administrators, Power Users, and Developers