See also Manually Add and Remove IP Addresses to a Mikrotik Blacklist

You can use DROP or REJECT depending on what you want the blocked host to know.

iptables -I INPUT -s nnn.nnn.nnn.nnn -j DROP

Delete the rules:

iptables -D INPUT -s nnn.nnn.nnn.nnn -j DROP

Show the rules:

iptables -L INPUT

One-liner to block evil hosts grep'd from Apache error logs:

for ip in `grep spammer.com.br /var/log/httpd/error_log|egrep -o "client [0-9]+\.[0-9]+\.[0-9]+\.[0-9]+"|sort |uniq|cut -f 2 -d " "`; do iptables -I INPUT -s $ip -j DROP; done

http://en.linuxreviews.org/HOWTO_stop_automated_spam-bots_using_.htaccess

vim .htaccess

# Block bots by User Agent string
SetEnvIfNoCase User-Agent "^Mozilla\/4.0 \(compatible\; MSIE 6.0\; Windows NT 5.1\; SV1\)" bad_bot
# Block empty User Agent string
SetEnvIfNoCase User-Agent ^$ bad_bot
SetEnvIfNoCase User-Agent "^AESOP_com_SpiderMan" bad_bot
SetEnvIfNoCase User-Agent "^Alexibot" bad_bot
SetEnvIfNoCase User-Agent "^Zyborg" bad_bot

<Limit GET POST HEAD>
Order Allow,Deny
Allow from all
Deny from env=bad_bot
</Limit>

service httpd restart