You can disable user accounts in Mac OS X Yosemite by going to "Users & Groups" in System Preferences. Once there, select "Guest User" from the Other Users list and un-check the "Allow guests to log in to this computer" check box. The status of the Guest User account switches to "Off" to indicate that is has been deactivated.
Locking User & Group Preferences
As the administrator, you can lock the settings in Users & Groups so that other users can't change them. Lock the settings by clicking on the unlocked padlock icon in the bottom left of the window. It changes to a locked padlock icon to indicate that the settings have been successfully locked. Once locked, the settings cannot be changed by anyone unless they click the locked padlock icon and enter the administrator's password. Once you have finished editing guest user accounts and editing other user accounts, lock the preferences to keep your changes secure.
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