If your DNS servers allow recursion an attacker can use them to perform large amounts of DNS queries through your servers and waste your bandwidth and waste server resources. Turning recursion off is very simple.
For Linux:
Open /etc/named.conf in your favourite text editor and modify your named.conf to reflect what is shown below.
options {
allow-recursion {
localnets;
};
Save your changes and restart bind.
For Windows with Plesk + Bind:
Log in with remote desktop. Click start and then click run. Type in: notepad "C:\Program Files\SWsoft\Plesk\dns\etc\named.conf"
Press OK.
Modify the Options instruction to look like this:
options { allow-recursion {localnets; }; };
Restart your name server!
You can check to see if it's worked by doing a DNS Report and ensuring that it says your dns servers are not open. This is a great resource for checking out other DNS related issues.