HOW TO: Disable The Guest Account in Mac OS X
Something that has always bugged me about Mac OS X is that when you connect to a remote 
computer you have the option of being a registered user or a guest. Anyone that is on your network can connect to your computer via guest access, though this only gives the user access to the drop box, they will stil be able to place files on your computer without your knowledge. Lucky for us, there is a way to disable the guest access but instead of clicking a box in system preferences you will have to edit a .plist file. Mac Tips & More: Disabling The Guest Account in Mac OS X