Here's the summary of what I can think of, with alternate methods in case the first one doesn't work with you:

• Debian kernel 2.4: In /etc/modutils, change the line alias net-pf-10 ipv6 to alias net-pf-10 off alias ipv6 off Reboot.

• Debian kernel 2.6/Ubuntu ("official" method)/Fedora Core (method 1): In /etc/modprobe.d/aliases, change the line alias net-pf-10 ipv6 to alias net-pf-10 off alias ipv6 off Reboot.

• Debian kernel 2.6/Ubuntu (alternate method): In /etc/modprobe.d/blacklist, add a line blacklist ipv6 Reboot.

• RHEL/CentOS: In /etc/modprobe.conf, make sure you have alias net-pf-10 off alias ipv6 off Reboot.

• RHEL/CentOS/Fedora Core (method 2)/Mandriva (method 1): In /etc/sysconfig/network, add a line NETWORKING_IPV6="no" Reboot.

• SuSE 9.0: In /etc/modules.conf, change the line alias net-pf-10 ipv6 to alias net-pf-10 off alias ipv6 off Reboot.

• SuSE 9.1/9.2/9.3/10.0 ("official" method): In /etc/modprobe.conf, change the line alias net-pf-10 ipv6 to alias net-pf-10 ipv6 install ipv6 /bin/true Reboot.

• SuSE 9.1/9.2/9.3/10.0 (alternate method)/Mandriva (method 2)/Slackware: In /etc/modprobe.conf, make sure you change alias net-pf-10 ipv6 to alias net-pf-10 off alias ipv6 off Reboot.

• Gentoo: In /etc/modules.d/aliases, make sure you change alias net-pf-10 ipv6 to alias net-pf-10 off alias ipv6 off Reboot.

NOTES:

• In distros using /etc/modprobe.conf, beware that you might get overriden by /etc/modprobe.conf.local or by /etc/modprobe.d/*.
• If you disable IPv6, I suppose you don't need tunnels either. You could also add there:
  install sit0 /bin/true
• A last "anti-IPv6" issue: don't forget the old trick of checking if your Firefox has IPv6 disabled:
  • CTRL+L, about:config (Enter)
  • Filter by ipv6.
  • Set network.dns.disableIPv6 to true.