Blocking users

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:

Odds are, it’s a spambot. But who can block a user, who decides who can block a user, what are users blocked for, and what recourse does a blocked user have?