Changes required in pure-ftpd configuration file: /etc/pure-ftpd/pure-ftpd.conf
# Don't allow authenticated users - have a public anonymous FTP only.

AnonymousOnly no

# Disallow anonymous connections. Only allow authenticated users.

NoAnonymous yes

Now restart pure-ftpd server

service pure-ftpd restart

If you want to do that for all active connection then you will need to stop pure-ftpd server and kill all pure-ftpd processes.

service pure-ftpd stop
ps uaxf|grep pure-ftp|awk {'print $2'}|xargs kill -9
service pure-ftpd start