Of course they are. They obfuscate the actual facts of a situation, promote emotional reactions to events that did not occur, or happened through other means. At best, they're a complete waste of everybody's time. At worst, they may incite violence and behavior that is unproductive or downright dangerous. Nobody wants to go to war over completely imagined circumstances.
Yes, conspiracy theorists are harmful to society because they make all of their followers doubt the truth. Take this election for example. Since Trump claims the election is rigged, a lot of his supporters will not recognize Clinton as their president. This is unfair and we should only hear the truth.
Conspiracy theorists are often talked about by society, but not in a serious manner. Most people use the theorists as an opportunity to learn a little more about the subject at hand and laugh at the theorists. Many people respond to the theorists with facts about the case which support the government. This helps inform the common population about the event.
Most conspiracy theorists are completely harmless. Some are representing a pernicious and, sadly common, lack of trust in the authorities to tell the public the truth about important matters and protect their best interests. Others are simply looking for a world of wonder outside of their everyday existence – for instance, UFO hunters. They aren’t doing anyone any real harm.