On-line, a troll is someone who posts things for the sole purpose of causing discomfort, anger or pain.
When they first appeared they were limited to discussion forums but with the rise of social media trolls are becoming a real problem for organisations that have a strong online presence. They can appear as serial complainers or as seemingly random presences attacking your organisation's good name.
It's important to realize that trolls are not people with legitimate complaints. Their sole purpose is to cause your organisation harm.
Sometimes it can be hard to tell the difference between someone with a legitimate grievance that needs to be addressed and a troll. One obvious sign is that most people with real grievances will give you a chance to fix their problem, or at the very least respond to them whereas a troll will continuously attack without pause. A troll will also refuse to accept or even totally ignore any response you send.
Whether that's because they feel they've been wronged somehow or simply because they feel like it doesn't matter; the best way to deal with trolls is always the same: don't feed them.
First off, you do need to answer them, but only once. You're not engaging in an argument, you're simply showing anyone else watching online that you're willing to address complaints that get sent your way. After that one time response cut off all communication with the troll. Make sure everyone in your organisation is clear that no one is to give any time or attention to them.
Trolls thrive on conflict and if they can draw you into an online argument they will. When starved of attention they usually move on within a few days.