Give specific examples of how you approached this issue. The main reason why people post negative comments is because they feel they are not listened to, so acknowledge the issue as fast as possible before it escalates, see things from their point of view, social forums may not be the best place to deal with a situation so you can offer to continue the conversation in a different platform (e.g. phone, email, existing online support forum), a sincere apology followed by fixing the problem should solve the issue. However, in the case of trolls who enjoy the notoriety it brings, the best long-term strategy is to ignore them.