How to disable 1 version of PostgreSQL server without uninstalling it

I have installed PostgreSQL 9.1 and 9.2 on my Ubuntu machine. The postgresql service starts both versions:

$ sudo service postgresql start
$ * Starting PostgreSQL 9.1 database server  [ OK ]
$ * Starting PostgreSQL 9.2 database server  [ OK ]

I'd like to start just 9.2 server but without uninstalling 9.1, is that possible?

Short version:
Replace auto with manual in /etc/postgresql/9.1/main/start.conf

Long version:

Each PostgreSQL cluster in Debian/Ubuntu has a start.conf file that controls what /etc/init.d/postgresql should do.

This is documented with pg_createcluster:

   STARTUP CONTROL
   The start.conf file in the cluster configuration directory controls the
   start/stop behavior of that cluster’s postmaster process. The file can
   contain comment lines (started with ’#’), empty lines, and must have
   exactly one line with one of the following keywords:

   auto
       The postmaster process is started/stopped automatically in the init
       script.  This is also the default if the file is missing.

   manual
       The postmaster process is not handled by the init script, but
       manually controlling the cluster with pg_ctlcluster(1) is
       permitted.

   disable
       Neither the init script nor pg_ctlcluster(1) are permitted to
       start/stop the cluster. Please be aware that this will not stop the
       cluster owner from calling lower level tools to control the
       postmaster process; this option is only meant to prevent accidents
       during maintenance, not more.