I want to restrict user access to a particular macro. How do I do that?

You need to go into the Tools>Security>User & Group Accounts menu and set passwords for your users. Once you have set these up, go into the Tools>Security>Permissions menu, where you will be able to individually set access to whatever forms, tables and macros you want!

I found the whole process needlessly complex compared to, say, Lotus Approach but there you go! If you're stuck look at http://msdn.mickey$oft.com/library/officedev/office97/ORKhtml/029.htm