Use Apache to prevent a PHP script from connecting to a specific domain

Here is an example of how to do this with htaccess. Note that it is relatively easy to circumvent the referer. The same website also has an example on blocking by IP.

Correction: I found this tutorial on IPTables. Perhaps that can get you started.

If the PHP script is accepting parameters in the URL such as http://www.example.com/thescript.php?somesite.com you may be able to create a re-write in your .htaccess so anytime that URL is loaded it could simply refresh example.com

Or use an iptables rule such as iptables -A OUTPUT -d 0.0.0.0 -j DROP where the 0's are put the IP of the domain you want to block.

Having the php script modified would only be a few bucks on most freelance sites since it's pretty basic to add an if conditional statement that says if the URL entered is somesite.com don't execute.

I know this is somewhat hackish but it is dead simple to implement. If the script is attempting to connect to a dns based url (not an ip address based url), you could simply set your host file to point to 127.0.0.1 for that dns name only. This assumes you have no intention on browsing that particular site.

# in /etc/hosts
127.0.0.1  somedomainIwanttoAvoid.com

As a bonus, you can setup a virtual host locally to catch these requests and handle them however you want.

Good-luck!