MacBook Pro Security – How to Wipe a Thoroughly Secured Old Computer

Although I have concerns about recommending this solution, you can change the access permissions for the Security (or Security & Privacy) preferences pane itself so that only the Administrator account, or members of the Administrator group, will be able to open that pane.
[ᔥ Owen Linzmayer: Require Password to Wake 2005]

I want to emphasize that I have not fully tested this solution and that you should proceed with caution and do your own testing before implementing it across an entire production environment. It seems innocuous enough, but experience has taught me that changes in places that Apple does not intend you control can sometimes have consequences.

You can set permissions for the pane using the terminal, or by navigating to the /System/Library/PreferencePanes folder:

Selecting the Security pane file:

do a Get Info using the menu, contextual menu or ⌘ + i and change the permissions for everyone to no access. You will most likely have to click the lock in the lower-right-hand corner and authenticate with your administrator password before you can edit the permissions.

As long as it is ok to forcibly log-out the user, you just need to to logon as the local admin using Apple Remote Desktop (available in the App Store) or ssh (through a terminal), and kill that user's loginwindow process. You must login to the computer and not simply "Share screen" from Finder. Then you will be able to login.

Here's a really easy-to-read article about this.

Of course, for either of those options to work, you have to enable Remote Login and Remote Management in "Sharing" under the System Preferences. The Sharing menu is shown below:

If you would rather use fast-user-switching, which I personally don't like because it can often really slow down your computer, then you can set up an "logout user after x minutes of activity" option in the System Preferences -> Security & Privacy, which will return you to a login screen. There may also be nice hacks if you search around, that allow you to get the user-switching icon or a keyboard shortcut setup to do a fast-switch, or trigger a switch or logout by tweaking a call to CGSession, although that may have been possible only in older OSX versions. There is a huge discussion of many other ideas in that last link as well.