Note that with PostgreSQL 9.0 (and probably newer) on Mac OS 10.6.8 (and probably older), in order to prevent the PostgreSQL server from auto-starting, you have to:

1. Go to /Library/LaunchDaemons/
2. Open com.edb.launchd.postgresql-9.0.plist in an editor as root(replace 9.0 with your correct version number), e.g. sudo vim com.edb.launchd.postgresql-9.0.plist.
3. Locate the line <key>RunAtLoad</key>
4. Change the next line from <true/> to <false/>
5. Save and exit.

That should do it. Also, if you want to stop the server immediately, without restarting, su to postgres and run /Library/PostgreSQL/9.0/bin/pg_ctl stop -D<your data dir>. You can find out which is your current data dir with ps aux|grep postgre.

Source: http://forums.enterprisedb.com/posts/list/2240.page#8321