[montsa007]

Go to this website http://ipinfodb.com/ip_country_block.php and follow the on screen instructions, should help.
Generate the htaccess file and add the code in your htaccess.

Also block Lithuania, Romania, Hong Kong and China (The most horrible bots reside there)

But its not bulletproof, these bots can come to your site via a proxy.

I find the Vbulletin's inbuilt blocker useless, there is a plugin called 'stop forum spam', something like that which once added blocks the bot registrations in real time (it checks the ip of the bot with a huge database and saves heck load of work)