How to disable guest browsing in chrome on Mac?
By piecing together information I found on various forums, I was able to come up with this method that worked on my Mac Mini running macOS 10.15 (Catalina):


Open the "Terminal" application. Use "Spotlight Search" (magnifying glass icon in upper righthand corner of screen) to search "Terminal" and click on the first result to open the application, or go to Finder --> Go --> Utilities --> Terminal.
Copy and paste the command below into the Terminal window and hit "Enter" on the keyboard.

defaults write com.google.Chrome BrowserGuestModeEnabled -bool false

Restart your Mac and the option for guest browsing will be gone the next time you open Chrome.