I believe the setting you're looking for is in the Options of the Code
Editor, not the main Access window. In to code editor, go to
Tools|Options|General tab.

However, the best way to keep the users from seeing the code is to put an
error handler in all of your procedures.

On Error GoTo LabelName
or
On Error Resume Next

For more information on using On Error, see this link:

http://msdn.microsoft.com/library/default.asp?url=/library/en-us/vbenlr98/html/v
astmonerror.asp
http://msdn.microsoft.com/library/default.asp?url=/library/en-us/vbenlr98/html/v
astmresume.asp

It's still in the startup options.  It's the 'Use Access Special Keys' check
box in the bottom left of the box.