The Guest User account on a Mac allows access to the Mac that might be unwanted. Follow these steps to disable Guest Access. Note that you must be logged in as an administrator or be able to authenticate as an administrator.
1. Choose System Preferences… from the Apple menu.
2. Click the Users & Groups icon.
3. If the lock in the lower-left corner is locked, click the lock to be able to make changes.
After clicking the lock, you will need to authenticate with an administrator’s username and password.
4. In the list of users, click Guest User.
5. Uncheck “Allow guests to log in to this computer”
6. Click the Show All button to save your changes.
7. Click the Users & Groups icon to confirm that the changes were saved.
Note that I have noticed the Guest User account become enabled without making changes to Users & Groups. This might be a bug in OS X.