Not explicitly having the RootLogin directive in your proftpd.conf is fine. As is using "BanOnEvent RootLogin ...".
Again, I must ask how, exactly, you are testing this configuration. Logging in as the root user, only once, is not enough, assuming you are using "BanOnEvent RootLogin 2/00:10:00 06:00:00". You would need to connect, attempt to login as root, then disconnect. Connect again, attempt to login as root, disconnect. Within 10 seconds. Is that what you are doing, or are you simply connecting one, attempting to login as root, and that is it?