iptables -A INPUT -p tcp –dport 80 -m string –string ‘WordPress’ –algo kmp -j DROP

or

iptables -N WordPress-PingVerify
iptables -I INPUT -p tcp –dport 80 -m string –to 70 –algo bm –string ‘GET /’ -j WordPress-PingVerify
iptables -A WordPress-PingVerify -p tcp –dport 80 -m string –to 80 –algo bm ! –string ‘User-Agent: WordPress/’ -j RETURN
iptables -A WordPress-PingVerify -p tcp –dport 80 -m string –to 300 –algo bm –string ‘verifying pingback from’ -j DROP
iptables -A WordPress-PingVerify -j RETURN

or

iptables -N WordPress-PingBacks
iptables -I INPUT -p tcp –dport 80 -m string –to 70 –algo bm –string ‘GET /’ -j WordPress-PingBacks
iptables -A WordPress-PingBacks -p tcp –dport 80 -m string –to 80 –algo bm ! –string ‘User-Agent: WordPress/’ -j RETURN
iptables -A WordPress-PingBacks -p tcp –dport 80 -j DROP
iptables -A WordPress-PingBacks -j RETURN