The Nature of Usenet

What is Usenet?
Usenet is a cooperative medium, created for discussion, that has grown into a place where people also share binaries. A Google search for the "history of Usenet" will give you several good links.

Why is there so much trolling and bullying on Usenet?
You can be a good Netizen or not. It's entirely up to you. Bullies have chosen to spend their time on Usenet pushing people around. They do it because they like it. You can be anything you want on Usenet. They want to be bullies. They will give you other reasons, usually trying to justify their outrageous controlling behavior by alleging some slight or transgression. They dignify their behavior as fighting for free speech, fighting against "netcops", fighting pedophiles, or some other cause. But the bullying comes first, and the rationalization comes second.

How anonymity on Usenet contributes to bullying behavior
In RL (Real Life), you bully someone at your own risk. If you misjudge your power to exhibit controlling behavior with the wrong person, you may well get your ass kicked. With the anonymity that Usenet provides, bullies become emboldened. When there are no negative consequences for even the most outrageous behavior, bullies become bolder still. If the only control is self-control, it is easy to spin out of control. Especially if it gives you an adrenaline rush.

Kooks, trolls, and bullies on Usenet
Some newsgroups maintain a history of kooks and trolls that frequent their newsgroup. If the information is current, a twit list can be useful for your kill file. The Troll FAQ is a good resource for learning about trolls and bullies. 

What about free speech?
Free speech or mandatory listening? When trolling and bullying behaviors overwhelm normal topical discussion, when you can't find anything topical for hours or days because of all the Off-Topic sniping, you can be sure YOUR free speech has been trampled by a gang of thugs with the Usenet equivalent of megaphones. Besides, free speech is a government thing. There is no government on Usenet.

But don't even trolls have rights? Rights? What rights? 
Usenet Bill of No Rights

What can I do? These people won’t leave me alone.
It's called cyberstalking. It's what they do. You can either leave the group or adopt other strategies.