Yes you can

. I've never used these settings myself so might not be much help but you have to edit XboxMediaCenter.xml to do this. Here's this bit you need to change.
QUOTE
<masterlock>
<!-- Set to 0 to allow infinite password retries in XBMC. Setting a positive !-->
<!-- number N will deny access to any locked bookmark if N consecutive bad !-->
<!-- passwords are sent for any locked bookmark. !-->
<maxretry>0</maxretry>
<!-- Set to 1 to allow XBMC Master Lock to shut off the XBOX if the user !-->
<!-- enters too many bad Master Lock password retries. Set to 0 to disable. !-->
<enableshutdown>0</enableshutdown>
<!-- Set to 1 to prompt user for mastercode when doing add/edit/delete on all !-->
<!-- shares using the context menu, or 0 to only prompt on locked shares. !-->
<!-- Useful to prevent unwanted changes/viewing of shares and user/pass info. !-->
<protectshares>0</protectshares>
<!-- Set to 1 to prompt user for the mastercode when XBMC starts! !-->
<!-- If the user enter a invalid mastercode XBMC will shut down. !-->
<startuplock>0</startuplock>
<!-- Set to 1 to prompt user for the mastercode when FilemanagerWindow will be opened !-->
<LockFilemanager>0</LockFilemanager>
<!-- Set to 1 to prompt user for the mastercode when SettingsWindow will be opened !-->
<LockSettings>0</LockSettings>
<!-- Set to 1-15 to prompt user for mastercode when HomeMediaWindow will be opened !-->
<!-- 1=Music 6=Music & Pictures 11=Music & Video & Pictures !-->
<!-- 2=Video 7=Music & Programs 12=Programs & Video & Music !-->
<!-- 3=Pictures 8=Video & Pictures 13=Programs & Pictures & Music !-->
<!-- 4=Programs 9=Video & Programs 14=Music & Pictures & Video !-->
<!-- 5=Music & Video 10=Pictures & Programs 15=Music & Video & Pictures & Program !-->
<LockHomeMedia>0</LockHomeMedia>
<!-- Set mastermode as follows: !-->
<!-- 0 = Masterlock always unlocked. This is the default if not specified. !-->
<!-- 1 = Numeric lock. Can be typed via on screen numpad or remote control. !-->
<!-- 2 = Gamepad lock. Enter by pushing XBOX controller button sequence. !-->
<!-- 3 = Full-text lock. Enter with on screen keyboard or USB keyboard. !-->
<!-- Bookmark lock settings are only valid if the mastermode is > 0. !-->
<mastermode>0</mastermode>
<!-- Set mastercode: where mastermode is set to value: !-->
<!-- 1: Enter numeric password only. Do not use spaces or other characters. !-->
<!-- 2: Enter gamepad button sequence using the following letter codes: !-->
<!-- A B X Y = Same as buttons on gamepad !-->
<!-- U D L R = D-pad Up, Down, Left, Right !-->
<!-- W K ( ) = White, Black, Left Trigger, Right Trigger !-->
<!-- 3: Enter HTML-escaped alphanumeric password. Make sure all characters !-->
<!-- are available on your language's on screen keyboard/USB keyboard. !-->
<mastercode>-</mastercode>
</masterlock>
It all looks pretty self explanatory, to lock my programs and my videos set lockhomemedia to 9 and to do settings set locksettings to 1. Probably best to do a Gamepad lock if you don't have a remote or keyboard otherwise it'll take forever everytime you want to get into my videos/programs.
You can also lock specific bookmarks (i.e. any individual part of my videos etc. - even the drives are bookmarks) if you want:
QUOTE
<!-- All bookmarks can be LOCKED by adding lockmode/lockcode values as follows: !-->
<!-- Set lockmode to: !-->
<!-- 0 = Bookmark not locked. This is the default value if not specified. !-->
<!-- 1 = Numeric lock. Can be typed via on screen numpad or remote control. !-->
<!-- 2 = Gamepad lock. Enter by pushing XBOX controller button sequence. !-->
<!-- 3 = Full-text lock. Enter with on screen keyboard or USB keyboard. !-->
<!-- lockcode: where lockmode is set to value: !-->
<!-- 1: Enter numeric password only. Do not use spaces or other characters. !-->
<!-- 2: Enter gamepad button sequence using the following letter codes: !-->
<!-- A B X Y = Same as buttons on gamepad !-->
<!-- U D L R = D-pad Up, Down, Left, Right !-->
<!-- W K ( ) = White, Black, Left Trigger, Right Trigger !-->
<!-- 3: Enter HTML-escaped alphanumeric password. Make sure all characters !-->
<!-- are available on your language's on screen keyboard/USB keyboard. !-->
<!-- Example: !-->
<!-- <bookmark> !-->
<!-- <name>SMB</name> !-->
<!-- <path>smb://DOMAIN;username:pwd@MyWindowsServer/movies/</path> !-->
<!-- <lockmode>1</lockmode> !-->
<!-- <lockcode>12345678</lockcode> !-->
<!-- </bookmark> !-->