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http://blamcast.net/articles/block-bots-hotlinking-ban-ip-htaccess

1) block 3 different IP addresses:
order allow,deny
deny from 192.168.44.201
deny from 224.39.163.12
deny from 172.16.7.92
allow from all




2) This will block any IP address starting with "192.168." or "10.0.0." 
from accessing your site:
order allow,deny
deny from 192.168.
deny from 10.0.0.
allow from all


3) code to block any specific ISP from getting access:
order allow,deny
deny from some-evil-isp.com
deny from subdomain.another-evil-isp.com
allow from all