Minecraft Forums

Quote from pennepasta

In the server.properties file, there should be a line that says "level type" like so:
#Minecraft server properties
#Thu Mar 22 16:21:37 PDT 2012
allow-nether=true
level-name=world
enable-query=false
allow-flight=false
server-port=25565
level-type=DEFAULT-------------If you want flat, set to FLAT.
enable-rcon=false
level-seed=
server-ip=
max-build-height=256
spawn-npcs=true
white-list=false
spawn-animals=true
online-mode=true
pvp=true
difficulty=1
gamemode=0
max-players=20
spawn-monsters=true
generate-structures=true
view-distance=10
motd=A Minecraft Server

Then, if you have already generated a regular world, delete the "world" folder. When the server restarts it will generate a new, flat world.
Hope this helps!

-Pasta