There has been some talk on this issue lately. I believe you basically have two options. You can put the code for the left nav in a custom feature (it's extrememely simple to do) and add it to pages where you want it to display and leave it off of pages where not. This is a little inconvenient if you have only few pages where you don't want it. The other option is to duplicate your skin and leave it off the second one and use that skin when you don't want the left nav. You then have to update both skins when you make changes but it's not too big a deal if you aren't frequently changing your skin.

John