There is now way from the native OS to lock ONLY certain applications.
You can find an application in AppWorld which does this...
1. If any post helps you please click the below the post(s) that helped you. 2. Please resolve your thread by marking the post "Solution?" which solved it for you! 3. Install free BlackBerry Protect today for backups of contacts and data. 4. Guide to Unlocking your BlackBerry & Unlock Codes