IPv6 issues

[Kvetch]

Hello, I am on a IPv4 network and we current do not have a IPv6 connection 
so we do not support it. We have recently installed BIND 9 on our DNS 
servers and I set my named.conf to listen-on-v6 to none. Well I have been 
receiving complaints from our internet users stating that they are having a 
huge lag time connecting to our sites and there machines are IPv6 enabled 
but they are not on a IPv6 line. At first I thought the issue was just their 
systems causing the lag since they systems were set to enable IPv6 but now I 
am not sure if it something I did wrong in BIND or is it on their end? A 
bunch of the users were on Ubuntu boxes which I know has IPv6 enabled by 
default. I told them how to disable it and I thought that fixed the problem. 
I am still getting complaints from users stating that they are having issues 
connecting to our domains and these complaints are coming from IPv6 enabled 
users. They are not on a IPv6 network but are using machines that come with 
IPv6 enabled like Ubuntu, Suse and I think some blend of XP SP2. Should I 
turn on the listen on v6 even if I am not using IPv6? Is the problem on my 
end or theirs, theirs right? If the problem lays on their end wouldn't we 
hear more about this problem?
Thanks,
Nick

Below is my snippett from my named.conf
options {
directory "/var/named";
pid-file "named.pid";
statistics-file "/var/log/named/named.stats";
zone-statistics yes;
listen-on-v6 { none; };
allow-query { our-nets; };
# allow-recursion { our-nets; };
blackhole { bogusnets; };