Some of my fans asked if they could form a street team on my behalf. It's run by a couple of fans, not me, but I'm privy to the group discussions and can approve or veto suggestions. They promote my books when they're released, talk them up to people, stuff like that, but I have been VERY emphatic that they're not to do anything negative towards readers, reviewers, or other authors. No responding negatively to reviews, no moving other people's books/swag/etc around, no spamming, etc. Basically, if they want to be on a street team representing me, they need to behave as if they were professionals. Anyone steps out of line and I catch wind of it, I'll be the first to loudly and publicly shut that shit down.
So far, the group has been amazing. They've been extremely positive, and have struck a pretty good balance with promotion (increasing visibility without overdoing it). I've been incredibly lucky that my street team is sane, and I haven't had any problems with them.
But yes, OMG, I've encountered some street teams that are obnoxious and downright destructive. A street team can be a very positive, effective thing, but an author needs to make it absolutely clear that nastiness and negativity will NOT be tolerated, never mind encouraged.