The fact that they were able to unban themselves shows that its not a problem with ulx.
What he says.
We don't discount your issue, and we empathize with your plight.
Unfortunately, we find time and time again that ULX isn't the root cause.
It makes it easier for those who have gained root access to your server to administer your server as easy as it does for you who are supposed to have root.
Make sure you're not storing rcon passwords in your config files.
If you enable rcon, always set it's password in command line startup (like nathan says.)