It is recommended that you prevent users from registering on your website with some odd usernames like:
- root
- admin
- administrator
- webmaster
To prevent this from happening:
- Go to Administer > User management > Access rules > Add rule
- Set Access type to Deny
- Set Rule type to Username
- In Mask type root
- Click the Add rule button
Repeat this for every username you don't want to be created on your site.