JSI Tip 3674. How can I disable the New, Delete, and Change buttons on Windows Explorer's File Types tab?

There is no Group Policy to prevent users from changing file associations via Windows Explorer.

You can prevent users from changing associations via Windows Explorer's Tools / Folder Options / File Types tab, by setting the NoFileAssociate value name, a REG_DWORD data type, to 1. To configure this setting for a specific user, use HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\Policies\Explorer. To configure the setting for all users of a Windows 2000 computer, use HKEY_LOCAL_MACHINE\Software\Microsoft\Windows\CurrentVersion\Policies\Explorer.

NOTE: This entry does NOT prevent users from using ASSOC and FTYPE to alter associations, and it obviously does NOT prevent alterations via the registry.