The Unwanted Visitor
macOS includes a feature designed to let friends or colleagues temporarily use your Mac without giving them access to your personal files. This is the “Guest User” account. In some configurations, particularly after setting up “Find My Mac” (which uses the Guest account as a honeypot to track a stolen laptop if the thief connects it to Wi-Fi), macOS might automatically enable and display this Guest User account on your login screen. If your Mac never leaves your secure home office, having a secondary, unpassworded account permanently sitting on your login screen is both an eyesore and a potential minor security vulnerability.
Removing the Guest Account
To clean up your login screen and stop macOS from automatically displaying the Guest User account, you must disable it in the Users & Groups preferences.
Click the Apple logo in the extreme top-left corner of your screen and select System Settings. Scroll down the left-hand navigation sidebar and click on Users & Groups. In the right-hand panel, you will see a list of all accounts on the Mac. Look for the entry named Guest User. Below its name, it will likely say “Enabled.” Click the small “i” (Information) button next to the Guest User entry. A dialogue box will pop up. Toggle off the switch explicitly labelled Allow guests to log in to this computer. You will likely be prompted to enter your administrator password to confirm the change. The Guest account will instantly vanish from your login screen.