The first step in finding out where does the problem come from is to use clean files. ( the original ones released ). People will most probably script backdoors into the shared files, so they can "hack" easy if they notice you are using his/her files.
But let's say you are using CLEAN files ( which i highly doubt if you didn't get the files 4 years ago ), the next step would be to disable Auction, which may be bugged.
If with Auction disabled, the next options would be:
- option 1: Disable Trading
- option 2: Make all important items bound
If even that will not work, disable/rescript the teleportation methods in game.
If no options are left, you're screwd.
*Keep in mind that client side has little control, and sending packets randomly will disconnect the user.