But who can block a user
who decides who can block a user
what are users blocked for
It appears that all blocked users so far have been spambots.
Adminship on the wiki and on lesswrong.com are separate (they run on different software, with different user databases).
Here are the lesswrong.com moderators, though I think their powers are limited to editing posts and such. [Edit: that was incorrect, see Alicorn's reply.] Promoting posts is done by people labeled Editors (shown next to their karma score on their user pages), currently Eliezer, Robin Hanson, and wmoore. (I'm not sure if any of them actually do any editing/promoting other than Eliezer.)
And I don't think the LW software supports completely blocking a user, currently.
Wikipedia's problems stem from a variety of issues, including seriously haphazard ways of determining what results in banning for anything other than absolutely cut and dried stuff. Wikipedia has a lot of different problems. The project succeeds primarily because the problems exist around the edge cases and the vast majority of editing never runs into them.
I noticed this recent Wiki edit:
http://wiki.lesswrong.com/wiki/User:AllisonGibbons
which says:
This user is currently blocked. The latest block log entry is provided below for reference: