Can I use Group Policy to prevent users from changing Microsoft Office 2003 Service Pack 2's (SP2's) anti-phishing setting?

No. The anti-phishing feature is turned on by default. If the user clicks Tools, Options, Junk E-mail and clears the Don't turn on links in messages that might connect to unsafe or fraudulent sites. To help protect your security, we recommend that you leave this check box selected check box, Outlook stores that preference in the user's mail profile settings in the Windows registry. Group Policy Objects (GPOs) can't manage mail profile settings without the assistance of a third-party tool.

Specifically, the registry setting that stores the user's preference is a REG_BINARY entry named 000b042a in HKEY_CURRENT_USER\SOFTWARE\Microsoft\WindowsNT\CurrentVersion\WindowsMessagingSubsystem\Profiles\profile name\0a0d020000000000c000000000000046. When the entry is absent or has a value of 00 00, links are disabled in suspect messages. When the entry has a value of 01 00, links in suspect messages are left active.