Nothing is more annoying than when something has to be done quickly and you can't find the darned instruction how to lock TYPO3 temporarily. Therefore to copy for you:
# Close for all backend users
$GLOBALS['TYPO3_CONF_VARS']['BE']['adminOnly'] = -1;
# Only admins may login
$GLOBALS['TYPO3_CONF_VARS']['BE']['adminOnly'] = 1;
# Only admins and CLI users may login
$GLOBALS['TYPO3_CONF_VARS']['BE']['adminOnly'] = 2;
Hinweis: You can put this statement in typo3conf/AdditionalConfiguration.php to prevent editors from logging into the backend.