Not sure if it's possible but you could use another email client as well as Mail. There are a few reviewed here. Perhaps a separate User Account for the whole machine would be the only way to make it fully secure.
Mark is correct. The way the Mac is designed, each user has full access to all of their files including all e-mail accounts. There are utilities available designed to password protect folders but I wouldn't trust this with an e-mail account. Setting up a separate user is the way forward.
Michael Corgan 