I’m not suggesting that you ignore upset customers or that you don’t respond to someone who questions you about something. Having this access to your company is a huge part of what is fantastic about social media. Respond to them. Engage them in a discussion. See if you can help.
The hard fact is, some people just want to cause problems.
No matter what: Do not take it personally in a negative way. Instead, consider it a compliment. You’ve created enough of a presence to attract their attention… CONGRATULATIONS!