How can I only allow a player to spawn once on a Minecraft server?
With this plugin for bukkit -> Exile v2.0 - Banish from the server or world on death!
You can set a timer on how long they can be banned before they can come back to the server or set it to eternity.
You can do this in a vanilla server now by setting this line in server.properties:
hardcore=true
This will auto-ban anyone who dies on the server.