Goto server.properties and change online_mode to false.
Server.properties will look something like this.
#Minecraft server properties
#Wed Feb 23 18:53:30 EST 2011
level-name=world
hellworld=false
spawn-monsters=false
online-mode=false <---------- if it is true change it to false.
spawn-animals=false
max-players=10
server-ip=
pvp=false
server-port=25565
white-list=false
spawn-protection=16