1- Change your rcon password.
2- Check on server.cfg and make sure that sv_cheats are 0. It MUST be 0 or you will continue to get hacked. That's how I got hacked in the first place. Never ever ever ever ever ever make sv_cheats 1.
3- Edit the clients.txt in your own computer, make it read only and rar it (after making it read-only) then upload it as a .rar (using the Game Panel provided by EndlessGaming.co.uk) and then once it's put into place, you should see clients.rar file. Unrar it from your Control Panel (there's an option for it) and restart your server.

Regarding the hacker's IP's, all you can do is ban him. When you ban him, do not include the "52656"part.

Please let us know how that works out for you. If you follow the instructions I gave, you should not have any problems because your clients.txt will be read-only. HOWEVER, you will have to do process #3 (edit it on your computer, rar, upload then unrar within the Game Panel) every time you need to add an admin. The way to check if it worked is, try to edit your clients.txt from within the CPanel. If you can't edit it, you will have succeeded. If you can edit it, it means you did not do it correctly.

Yes, the process is tedious but it will get you out of this problem and add a lot of protection.