You can also ban their IP address using .htaccess so that they will not be able to access your web site. Just add following line in your .htaccess: deny from x.x.x.x Kailash
Use IP banning cautiously. My real job is in a University. 30,000 of us are on the same IP address or close IP's. I was recently banned from a forum that I have never posted at! Out of the 30 000 how many are active in forums?