Disable IPV6 module on default kernels

Debian Etch (default kernel version 2.6.15-1)

Where?
/etc/modprobe.d/aliases

By default you will have a line like this: alias net-pf-10 ipv6 in the aliases file. Replace that line with:
alias net-pf-10 off

RHEL4/Centos4 (default kernel version 2.6.9-34.EL)

Where?
/etc/modprobe.conf

Add the following line to the modprobe.conf file:
alias net-pf-10 off

On your next reboot the IPV6 module will no longer be loaded. In a similar way, you can disable other kernel modules from autoloading, based on your particular needs. See the modules documentation for further details.