irule to disable interface

How often are you wanting to run this? If it's on a per minute basis you could just add a crontab with a shell script like this:

!/bin/bash

count=0
IP_ADDR="1.2.3.4"

count="`ping -c 1 $IP_ADDR | grep 'packets transmitted' | awk '{print $4 }'`"

if [ $count -eq 0 ]
then
    echo "$count packets received"
    echo "ifconfig interface1 down"
fi

Or you could make it an infinite loop with a sleep period.