How to turn on/off certain features left navigation?

This has to be done by App Manager.

To change the left hand navigation options:

1. From "Manage Application", go to "Set up" > "Navigation".  Select the items you want to appear on the left hand navigation. You can also drag and drop to rearrange the options.
2. You can also add up to 3 external links to your left hand navigation.
3. Click "Save".