Deny login to Panel with username root.

[mezzomix]

I just installed Plesk Panel 10.0.1 with the Power User Panel. I can logon as 'admin' and as 'root' at https://domain.com:8443.
It looks like user 'root' is the debian user 'root' and user 'admin' is a plesk user but not a debian user. Could someone please clarify that?

But what I actually want to do is to allow login only for 'admin'. Can anybody help me here?