The correct command is userdel -r username. The -r option removes the user's home directory and mail spool along with the user account itself. Without the -r option, the account will be deleted, but the home directory and mail spool would remain on the system. The usermod command is for modifying an existing user account, not deleting it. The deluser command does not exist by default in most Linux distributions, and userdel --remove-all-files is incorrect because the --remove-all-files option is not a valid option for the userdel command.