Unknown bots searching my sites... please help

I have pdf search engine.Some unknown bots are searching my site with same search term so many times.That is definitely not a real visitor.

Please help me how can i stop this. These are some of the urls.

http://ggiaro.com/ebook-1+email+@yahoo+com+@hotmail+com+@msn+com+@gmail+com+@aol+com.xls-txt.html

http://ggiaro.com/ebook-966+@hotmai...d+!!!!+2009+!!!!+Email+Leads!!!!+txt-pdf.html

My site is in my sig 1..

Please help me.
Thanks

 

Maybe you can check IP addresses where this requests are coming from in your web server logs, and block them with a firewall.

 

everytime is using different ipaddress... Anyway i solved it in some otherway..

Thanks