Open registry editor and browse to HKEY_LOCAL_MACHINE\SAM\SAM\Domains\Account\Users\Names
, delete the Guest key and reboot your computer. You'll see the Guest account was deleted and you can't access it any longer.
Source: https://www.top-password.com/knowledge/delete-windows-built-in-administrator.html
Personally I suggest you make a restore point before deleting the guest account, in case you mess up the registry.