If you are using a custom form in Outlook 2002, 2003 or 2007 and don't want your default signature added to it, you'll need to create a registry key to block it.

Browse to HKEY_CURRENT_USER\Software\Microsoft\Office\xx.0\Outlook\Options, where xx equals your version number of 10, 11 or 12. Add a DWORD of NoAutosigOnCustomForms, with the data value of 1. Restart Outlook and open a custom form to verify your signature isn't added.

If you want the signature added to some forms and not others you'll need to do it the old fashioned way - either leave this enabled (value of 0 or delete the key) and delete the signature or use the key with a value of 1 and insert the signature when needed.