I tried safariblock before and I was not impressed- it seemed to slow my system down.. I tried glimmerblocker its OK but its too complicated to add a rule to block a specific site.
I finally figure out how to use pith helmet in safari 4, go to (this is not in your home folder!)
Library/Application Support/SIMBL/Plugins/PithHelmet.bundle/Contents and edit the info.plist file with TextEdit
and
look for <key>MaxBundleVersion</key> there is going to be a number with it. change that number to 5528- works like a charm! remember to save it!
Good luck