Hide/Remove a Key from Keychain Access
My first idea is to use Parental Controls to restrict access to Keychain Access:

System Preferences > Parental Controls > Add the account that will be used > Apps > Limit Applications > Check all the boxes except Utilities > Keychain Access

If you need the users to be able to open Keychain Access (For example, to manage their own keychains), you can set up the login keychain so that it doesn't unlock automatically and allow/require end users to create and use non-login keychains.  To set the login keychain so it doesn't automatically unlock, open /Applications/Utilities/Keychain Access, go to Preferences > First Aid, and uncheck the last three checkboxes, "Synchronize login keychain password with account", "Set login keychain as default" and "Keep login keychain unlocked".

Neither of these are exactly what you're asking, but should solve the problem.