You need SSH access to your Plesk panel to do this.
Login with your account with SSH or with su as root.
Use your favourite editor (i use Nano) and go to
nano /etc/drweb/drweb32.ini
look for
CronSummary=yes
and change it to
CronSummary=no
Also you can edit the
nano /etc/drweb/drweb_handler.conf
where you can look for the Notification section and change anything that you dont want to be notified about by simply putting no instead of yes
After any changes you need to stop and start the drWeb Service.
you can do that by
/etc/init.d/drwebd start
/etc/init.d/drwebd stop