Slipstick (uma ótima referência em todas as coisas do Outlook) tem uma explicação sobre como limpar toda a lista. Aparentemente, não há como remover entradas específicas:
To clear the Location drop-down list on Outlook appointments
Outlook appointments show a drop-down list for Location, populated with the most recently used locations added to appointments. You cannot populate this list with your own preferred locations. You can, however, clear the list by closing Outlook, removing a value in the Windows registry and then restarting Outlook. Always make a backup of the registry before making any changes.
For Outlook 2000, the value to remove is:
HKEY_USERS\.DEFAULT\Software\Microsoft\Office.0\Outlook\Preferences\LocationMRU
Outlook will automatically add the value back into the registry the next time you set a location on an appointment.
In Outlook 2002, 2003, and 2007, the value is found at:
HKEY_CURRENT_USER\Software\Microsoft\Office\xx.0\Outlook\Preferences\LocationMRU
Replace the xx with 12 when using Outlook 2007, 11 when using Outlook 2003, or 10 when using Outlook 2002.