FOSS gamedev and creative worlds

When I try to ban a player from my server, it just prints out "Server:" and the player doesn't appear on the ban list. Does the player have to be online?

Yes, currently players have to be online for it to work, because the /ban function only reads the IP that the player is using at that time, which is null if they aren't online. Banning people that are offline would require listing IP's used by players, which is not possible unless the game itself is changed.