You can set up an .htaccess file to deny access from a hostname or an IP address.
Look in your httpdocs folder for a file named .htaccess. If you do not have this file create a file with the name .htaccess with the below information; If you already have a file named .htaccess add this to the bottom of the .htaccess file:
deny from 126.96.36.199
deny from example.com
allow from all
where 188.8.131.52 and example.com are IPs and/or hostnames you wish to block. You can add additional "deny" lines for sites that you do not want to be able to reach your domain. Place them below the last "deny" line but before the "allow from all" line.
Once you have added all the "deny" lines you need, save or copy the .htaccess file to your httpdocs folder.
For more detailed information on Apache directives see http://httpd.apache.org/docs/2.0/mod/quickreference.html