From brokenpoet.org wiki

Thanks to Steven King for this bit of info . . .

In order to lock a domain, you must make an entry in the named.conf for each domain you wish to lock.

The entry is:

allow-transfer { none; };

For example:

zone "domain.com" IN {
        type master;
        file "domain.com.db";
        allow-transfer { none; };
};