After you apply SQL 2005 Cumulative Update 3
(3186), members of the processadmin fixed server role can no longer kill spids (which since that is all that role allows you to do effectively makes it useless). This is supposedly fixed in Cumulative Update 4
(though I haven't tested it myself). What is annoying is that there is no mention of this that I can find in the KB article for Cumulative Update 4
so the only way to confirm the bug and find out that it got fixed is to raise a case with PSS. There needs to be more transparency when regression bugs are introduced and then silently fixed.