The entries in the rc.n directories are aliases (softlinks) to the master scripts. You want to remove both the SnnServicename and the KnnServicename from all the numbered runlevel directories.
In Red Hat/Feodora, there's a magic comment in the initscript that specifies run level information (it's also done in SuSe, but slightly differently). You can turn services on and off via a gui command or via the chkconfig utility, like so:
chkconfig --level 345 postgresql off
Occasionally, Ubuntu adopts Red Hat stuff, so you might want to see if that works.
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