"I have always relied on the kindness of strangers" is one of the most famous lines in Tennessee Williams' play A Streetcar Named Desire.
Spoken by his heroine Blance DuBois, it's exactly what we too often ask brands to do in the social media arena. Take the whole AMC/Mad Men/Twitter exercise. Now that AMC is letting the faux Sterling Cooperites run with their characters, they are taking a leap of faith that the people playing those characters will act in a way that portrays the brand in a positive light and won't offend anyone.
It's a smart decsion on their part, because the last thing these avid fans would want to do is to hurt the show. They've also proven themselves to be fairly tame and true to character over the past several days.
But Mad Men is a popular TV show and the people in question are just play-acting. The situation gets stickier when you're talking about companies and people posing as company spokespeople (as in the Exxon case a few weeks back.) There, your best bet is to shut them down, even if they don't mean any harm, because there's nothing to be gained and much to be lost via an inadvertent slip of the tongue.
I don't know that there's any hard and fast rule on how to handle these situations. With Mad Men, I've seen people suggest that AMC should have created the characters themselves.
But that still wouldn't prevent an @ message aimed at one of the characters with content some group might take offense at. (Which, in my opinion, is a risk worth taking.)
Social media is a messy place where brands don't get to control what's being said of them. All they can really do is trust that the people who say they are fans really are and, like Blanche DuBois, rely on the kindness of strangers.