Outlook stores a number of recently used lists, from other users mailboxes, to locations, and recent searches, and stationery lists. If you want to clear a most recently used list you need to delete the appropriate registry value. Because the values are stored in binary format, you can't easily edit the list to remove some entries from it, only delete the value, which deletes the entire list.

In most cases, Outlook needs to be restarted for the changes to take effect.

While most of the keys and values are the same between versions (except for the moved Profile key), the MRU lists for older versions of Outlook are at Clearing Outlook's Most Recently Used (MRU) lists.

In Outlook 2016, the MRU lists are all stored in the Outlook key. (In Outlook 2013 and above the Profile key in under the Outlook key.)

HKEY_CURRENT_USER\Software\Microsoft\Office\16.0\Outlook\Profiles\profile-name\0a0d020000000000c000000000000046

The MRU's that affect all profiles are in
HKEY_CURRENT_USER\Software\Microsoft\Office\16.0\Outlook\

Outlook 2013

HKEY_CURRENT_USER\Software\Microsoft\Office\15.0\Outlook\Profiles\profile-name\0a0d020000000000c000000000000046

The MRU's that affect all profiles are in
HKEY_CURRENT_USER\Software\Microsoft\Office\15.0\Outlook\

MRUs under the Profile key

The values in this table are all stored in the 0a0d020000000000c000000000000046 key under the Profile.

MRUs under the Outlook key

Navigate to :
Outlook 2016
HKEY_CURRENT_USER\Software\Microsoft\Office\16.0\Outlook\
Outlook 2013
HKEY_CURRENT_USER\Software\Microsoft\Office\15.0\Outlook\

The Fonts MRU list is stored in the SettingsWordMail value in HKEY_CURRENT_USER\Software\Microsoft\Office\15.0\Word\Data. I'm not sure exactly what else is in the value, but you need to delete the value to clear the Fonts MRU.

Do It For Me

If you don't want to edit the registry yourself, you can download and run the reg file for the key you want to delete. Make sure you choose the correct version and remember to restart Outlook for the key to take affect.

Appointment Locations (LocationMRU): Outlook 2013
Advanced Find's Search for Words (MRU 1): Outlook 2013
Advanced Find's More Choices, Categories (MRU 3): Outlook 2013
Choose Form dialog (Office Explorer \ Namespaces): Outlook 2013

How to use the Registry Editor

Many of the Most Recently Used (MRU) lists in Outlook can be deleted by editing the registry. Quit Outlook then open the registry editor from the Start menu, Run command by typing regedit, and then click OK.

Navigate the left pane of the registry editor, looking for :
HKEY_CURRENT_USER\Software\Microsoft\Office\15.0\Outlook\

Once you locate the proper set of keys, use File, Export to back up this section of the registry. If you are worried you made a mistake, you can double click on the *.reg file to restore it. Next, browse for the specific keys mentioned above. Select the value on the right side and delete it.

MRU's in other applications

If you need to delete MRU list in other windows applications use the Registry Editors Edit, Find menu to look for the keyword "MRU".