You can use IP Tables or a reject route.
Reject route ex:
/sbin/route add -host 200.x.x.x reject
list routes: route -n
remove block: route del 192.168.0.123 reject
or, you can use iptables to ban a range of IPs from your server, like so:
iptables -I INPUT -s 126.96.36.199/188.8.131.52 -j DROP