Here's a backup article:
So it's not that simple, nor complicated. Just tedious. Some Deletions will cascade and wipe out users, databases, and one-click installs within a domain, so deleting entire domains will save you some other steps.
However, most will argue this is a bit of overkill if the problems are narrow in scope.
To start wiping stuff, you'll have to go to each section and click on the X to get rid of stuff from the Panel:
Goodies->One Click Installs