I know i asked this question early last year but i thought there may have been an update since then.

I want to ban members from my site. How can i do this...?

The most complete way to do it is to find out their IP address (from a news comment or the Who's Online block, maybe) and add it to the banned IP list in the general preferences.

I took a somewhat different approach on my site. I created a group called "Trolls," which has access to only the basic website modules, and an additional block explaining that they've been restricted due to misbehavior. When somebody misbehaves, simply add them to the Trolls group and remove them from the Registered group.