With external recursion enabled, a DNS server can be exploited to attack other networks. You can disable recursion in BIND but making a change to the config file.
Using your favorite text editor, open the file /etc/named.conf
nano /etc/named.conf
Look for a line that says:
recursion yes;
Change that line to no:
recursion no;
This will disable recursive lookups and protect your server from being used to attack other servers.