I'm not terribly experienced with UEC however I would wager some of the typical methods would work.
Have you simply tried to userdel the ubuntu user. Of course making sure you new admin user is a sudoer.
Note : you can insure your new user is an admin by doing the following.
sudo usermod -G admin youradminusername
Alternatively... If you can't delete it (I wouldn't see why not) you could always just lock the account, again making sure you new user is a sudoer.
sudo userdel ubuntu
Third if none of this works remove them from the admin group
sudo usermod -L ubuntu
Alternatively if none of these work some good old brute force protection methods like using keys in conjunction with iptables rate limiting (won't stop a brute force but will make it so slow as to be ineffective).
sudo usermod -G somenonadmingroup ubuntu
Also... You could probably rename the ubuntu user.
Should get you where you want to be.
sudo usermod -l newusername ubuntu
I'm not too sure about the webmin stuff as I don't use it, but I know many people on here do so I'm sure you'll get an answer to that soon.
Hope this helps.