In some situations, a guest account is useful, but most likely you will want to disable it to prevent people from using your computer without a password. There isn't a graphical way to do disable it. You must edit the lightdm.conf file to disable it.

Press Alt + F2 to open the run box and type in this:

gksudo gedit /etc/lightdm/lightdm.conf

Add the following line to the bottom of the file and save it:

allow-guest=false

Restart your computer and you will no longer be able to login as guest. To re-enable the guest account, just remove the line you added from the lightdm.conf file.