I know there's a wiki entry http://wiki.debian.org/DebianIPv6#How_to_turn_off_IPv6 that shows how to disable IPv6 in Debian Squeeze, but that's a 2.6.* kernel.
What about 3.2.* kernels running Wheezy?
Disabling IPv6 on more recent systems works the same way as described in the Debian wiki:
Keep in mind that, whatever problem you are having, this should be an absolute last resort after you have exhausted all other options for fixing the problem. IPv6 is not optional and you will have to turn it back on sooner or later.