Why is postgres autovacuuming a table that is never updated?

I'm running 8.2.6.  I have a log table that is always INSERTed to, and
no updates or deletes are ever run on.  For some reason the autovacuum
decided it needed to vacuum this table, and it is slowing down my
production server.

So my questions are:
  1) Why vacuum, if this table is never updated?
  2) How can I use pg_autovacuum table to disable autovac for this
table?  The docs are not clear on how to do this.