macOS creates the Guest User account. It’s a restricted account with no access to any documents and can’t save files. It’s perfectly fine where it is. When you install macOS, the deleted user will be recreated.
How to Turn Off guest user / any users from MacOS guidline
How to Delete a User account on macOS Sierra
Begin by opening the Search program, clicking File, and then finding “netplwiz” in the Programs box. Then hit Enter to execute it as an administrator. You’ll see a list of users after that. If Guest is on the list, remove it.
The –force (-f) switch, for example, tells userdel to forcefully remove the user account even if the user is still logged in or if there are still running processes that belong to the user.
nYou’re in a bind, but you’re not alone. The “nobody” user is common; it’s one of the five accounts generated by OS X. Your Mac has not been hacked.
To remove a user, use the deluser command. Usermod is advised by many people, but deluser is preferable since it has more options and more powerful functions. nDeluser offers further configuration possibilities and more effective tools than userdel. The system Delete only if the user is a system user.
To remove the built-in Administrator account, right-click it and select Delete. Close Registry Editor and restart your computer. You’ll notice that the built-in Administrator account has been removed from the Local Users and Groups window.
Disabling Guest Accounts on Windows Is Vital for Security. A user who is logged into a guest account on a Windows system that belongs to an employee at the organization might gain access to everything on that computer, including files, shares, and applications.