Closing oracle server when users are connected to database.

Hello friends,
  We have our Oracle database on a SUN Solaris machine. When I want to shutdown the database it doesn't shutdown if some user is connected to it. I tried the NORMAL & IMMEDIATE shutdown, but tthe commands did not shutdown the database.The commands produced no response. There is one more ABORT shutdown and I am not aware of its effects. So can someone please guide me as to how can I shutdown the database when the users are still connected to it. Thanks
Dhanesh

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