It's possible to enter 'any' in this field, but the effects are not all that useful. It will dissallow access if any username is set at all. If the 'Allow usernames' is set, no access will be possible at all. If not, access will be permited if no username/password pair has been entered by the user.
There is no way (atleast in Netscape or Lynx) to clear the 'send password to this server' flag, except quitting.