You'll probably find that these messages are system logs, and you're probably running as 'root'. Please don't use the root user as your normal user ID, since you can accidentally destroy your system, as the root user has "God" powers -- you can delete or change any file in the system. Trust me -- been there, done that! If you haven't already, create another regular user and login that way to use the system. Use root only to administer things that can't be done by a regular user.
OK, that's out of the way! To delete mail, enter the "mail" command, at the prompt enter 'd #- #' (without quotes, of course), where '# - #' is the range of messages you wish to delete. In other words, to delete the first 400 messages, you would enter 'd 1-400'. To quit, enter 'q'. (At least, that's the way it always used to be!)