Hi, the easiest way to disable IPv6 in Vista/Win7, if you don't use it at all, is disabling it right on the network adapter.
1) open the network control panel (e.g. by start/run and typing: ncpa.cpl)
2) find your local area connection (or Wireless Network Connection adapter - or both) and click properties
3) in the list of installed services choose Internet Protocol Version 6
4) uncheck the checkbox right next to it
5) click ok
This way you'll disable IPv6 on that adapter.
In avast firewall you can do it in the packet rules. Open the packet rules dialog, click add new rule, choose protocol 41, direction BOTH, action: DENY, click OK. Should be done.
I am afraid I must here confirm the bug on the dialog - it will display ALLOW again when the dialog is reopen, even when the rule is correctly set to block. This is a new present from the GUI team introduced with the latest program update. Its just the presentation layer (I mean, the rules are ok, just the dialog does not show it correctly) but I understand that does not makes much difference to you, guys. Sorry.