Create a .htaccess file and add the following code--changing the IPs to suit your needs--each command on one line each:
deny from 126.96.36.199
deny from 012.34.5.
allow from all
You can deny access based upon IP address or an IP block. The above blocks access to the site from 188.8.131.52, and from any subdomain under the IP block 012.34.5. (012.34.5.1, 012.34.5.2, 012.34.5.3, etc.)
You can also set an option for denying from all, which would, of course, deny everyone. You can also allow or deny by domain name rather than IP address (allow from .mysite.com works, etc.)