I know this isn't related to bukkit, but I thought I could get some help. One player, Dominik813, has claimed to be a hacker, which I don't know if he is. He is also a griefer. I've tried to get him off my server for a few days, and even though my server has a whitelist, he can get on. He has claimed to be a hacker today and after I killed him, he left the server and, not 5 minutes later, my server crashes.
I think "/#ban Dominik813" will add him to ban file. But yeah, minecraft client and server seems to be very insecure. Server inventory fixed at least item hacking, but there is still big potential to hack. Minecraft needs a lot of work on the security. I guess simple plugin that will kick him each time he joins, will make workaround to the whitelist problem. As for crash, I'm not sure.
I don't know how the Authorize forums would help, as there is nothing wrong with it. But ill try banning his ip with a command I guess, I never thought they had a built in banning system.