The existing /usr/sbin/ppp-off script should work just fine when run as user root. The only changes you may wish to make are for the script to wait for any outgoing email currently being processed by sendmail.
This is left as an exercise for the student!
In addition, you can create a script file that will be executed once the link has been terminated. This is stored in /etc/ppp/ip-down. It can be used to undo anything special that you did in the corresponding /etc/ppp/ip-up script.