Here's a new website called www.boycottmillerbeer.com. They promise that they'll shortly have a complaint form of some kind to send emails or similar to Miller; it's apparently from one Leslie Wetzel, who's a member of U.S. Border Watch and TX MM, among other groups.

UPDATE: Here's another: millerboycott.com. It's run by alipac.us and 100 other groups and appears a bit further along than the other site; I'd suggest concentrating on this one.