How to move players from a world to another after logging in?

Hi! I'm new here at Bukkit Forums. If this thread is considered to be in other section of the forum, please tell me.

I'm doing a Minecraft Factions Server 1.8 Offline Mode (Cracked), and, has the title says, I would like to move players from a world to another after loggin in.

Basicly, in cracked servers, you can pick a name that you want (incluiding other account names). That means that if one of the players that are in my server log out, another player can access his account by opening another minecraft launcher, putting the name of the player that left the server, entering the server and get his coordinates where he left (let's say, in this case, his secret faction base). Eventually, this player raids the faction base and when the player gets back, POOF, everything destroyed.

I want the players to be safe in this example, by making a world where everyone has to log in and once logged, they automaticly get teleported to the coordinates they left on the other world (factions, in this case).