Open System Preferences
(Located in Applications OR open with --> Sytem Preferences...)
Under System (4th Group) click on Users & Groups.
In the lower right corner click on the lock and enter your user password.
After unlocking click on the Guest Account in the right sidebar and disable it by removing the check mark next to the "Allow guest to log in to this computer".
Hope this helped!
Note that, if geohount's solution doesn't work for you, it's probably because you have turned on Find My Mac in th iCloud pane of System Preferences. With Find My Mac on, the Guest account will be enabled regardless of the setting in Users & Groups, because otherwise there is no way for someone to connect to a wireless network and use the machine, without which Find My Mac is useless.
Out of curiosity, why do you want to get rid of the Guest account? It can't hurt anything to leave it enabled. Are you under the impression that doing so is a security risk?