how to secure ntpd

In the ntp.conf

restrict default limited kod nomodify notrap nopeer
restrict -6 default limited kod nomodify notrap nopeer
restrict 127.0.0.1
restrict -6 ::1

Add noquery if you did not like to query.

Further information http://support.ntp.org/bin/view/Support/AccessRestrictions#Section_6.5.1.