While some may find this feature useful, I find it annoying and infuriating, since I only use Notepad++ for basic text editing. It's a great program for that, but that stupid edit margin that showed up a few versions back is a distraction. And nobody here has been able to tell me how to disable it.
Well, I finally figured it out, and I'm quite proud of myself for it. So here it is for anyone else bothered by it.
Plugins > ChangeMarkers > Disable Plugin
It's that easy. Also, if you find this feature useful, go into that submenu; there are other options you can set. See About for more info.
Hope this helps!