70% of the bandwidth is used by 2 adjacent IPs:

194.83.245.140
194.83.245.141

Now what do I do to prevent them from abusing my proxy? Do anyone of you have proxy sites? What do you do to prevent spam requests?

in your apache .htaccess:

<directory />
  order allow,deny
  allow all
  deny 194.83.245.140 194.83.245.141
</directory>

This should prevent those 2 from accessing your site.

Firewall ?

This should prevent those 2 from accessing your site.

Thanks! rep added.

And is it also effective in preventing google bot from indexing my site?

Nope, you need to make a robots.txt file in the root of your site to do this.

Content of which should be same as here

http://www.robotstxt.org/faq/prevent.html

Originally Posted by tripy

in your apache .htaccess:

<directory />
  order allow,deny
  allow all
  deny 194.83.245.140 194.83.245.141
</directory>

This should prevent those 2 from accessing your site.

does that really work? what happens when they access the site?

They get a 303 error: access denied (I believe...)