are you on a minilogin?
it's been awhile since i've run a server, but lemme throw some ideas out.
1) You're using a minilogin. In mysql you gave your IP access of 250, but somewhere your IP's are crossed and it's giving your entire network access of 250.
However, it doesn't sound like you're using a minilogin.
2) similar user names? i remember a .ini called accounts that was bundled with the minilogin. Might want to look at that.
3) It seems unlikely that he's using a hack program. To me it would be more likely that he has access to your db.
If i were you, i would find him and ban him.
4) Could change it so everyone has access of 1. If he does have a program, then maybe it will throw it off.
5) Check to make sure you do have everyone's access to 0. Mysql never seemed that straight forward to me, perhaps someone here can help you clear all the permissions.
Look at your output for minilogin, world, and zone exe's. There might be a hint in there at what the problem is. If it does seem like the problem is on your end, it couldn't hurt to post the ouput of the exe's.
6) change the password to your db.
7) if you use a peer to peer network (Kazaa, limewire, ect.) check what files you're sharing.
Also check what commands you have given to those with access of 0.
Good Luck.