Why on EARTH would you save the banlist to THEIR computer? They can just delete it and rejoin! >_>
You don't save the banlist to their computer, you save a specific file that only bans them. 99% of the noobs here on BYOND would never find the thing, even if they were looking for it.
and @ Axiom:
I'm still pretty sure client.Export() saves on a per-key basis. So unless you know of some other way to save files to the player's computer; it would only work on 1 key.
[EDIT] I found the appropriate term. It is called a hash. You save A (Just one) .sav file to thier computer. Then check for that .sav when a player goes to log in, if it is there just add him to the ban list and kick him. The ban list is perfectly safe on wherever you curently have it.