no more recursive-clients

[Duane J. Von Lanken]

I am getting the following error on a DNS caching server:
Oct 29 12:31:25 ns1 named[21440]: client 209.175.98.2#39641: no more
recursive clients: quota reached
I have raised the recursive-clients running on Bind 9.2.0 to 5000:
options {
        directory "/etc/named.data";
        pid-file "/var/run/named.pid";
        version "version id withheld";
        datasize 256m;
        max-ncache-ttl 60;
        recursive-clients 5000;
};
What else can I do to stop the server from getting the quota reached
error?
How do I check/stop a DoS if that is what is happening?