I agree with Ulrich, that this doesn't sound like an IPv6 problem. However, here's how to disable IPv6.

In sysctl.conf set the following options:

net.ipv6.conf.all.autoconf = 0
net.ipv6.conf.all.accept_ra = 0
net.ipv6.conf.all.disable_ipv6 = 1

I agree with Ulrich, that this doesn't sound like an IPv6 problem. However, here's how to disable IPv6.

In /etc/sysctl.conf set the following options:

net.ipv6.conf.all.autoconf = 0
net.ipv6.conf.all.accept_ra = 0
net.ipv6.conf.all.disable_ipv6 = 1

If you don't have /etc/sysctl.conf just create it and add those lines, then reboot.

Alternatively, each of these has an interface in /proc that you can flip (and/or create a script to do this at boot time).

echo 0 > /proc/sys/net/ipv6/conf/all/autoconf
echo 0 > /proc/sys/net/ipv6/conf/all/accept_ra
echo 1 > /proc/sys/net/ipv6/conf/all/disable_ipv6