How to disable IPv6 on Debian Wheezy?

If you do sysctl -p, the reboot isn't necessary. It worked for me, but I prefer to keep params in single file, so I put the line above in the sysctl.conf file.

Just to be sure, I put a line about every device, so my solution is adding the following lines to /etc/sysctl.conf:

net.ipv6.conf.all.disable_ipv6 = 1
net.ipv6.conf.default.disable_ipv6 = 1
net.ipv6.conf.lo.disable_ipv6 = 1
net.ipv6.conf.eth0.disable_ipv6 = 1
net.ipv6.conf.eth1.disable_ipv6 = 1
net.ipv6.conf.ppp0.disable_ipv6 = 1
net.ipv6.conf.tun0.disable_ipv6 = 1

Disabling IPv6 on more recent systems works the same way as described in the Debian wiki:

Disable ipv6 in kernel : echo net.ipv6.conf.all.disable_ipv6=1 > /etc/sysctl.d/disableipv6.conf will disable ipv6 at next reboot.

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.