Prevent connections for some users through SQL*Plus, TOAD

Hello to you all,

I've an old fashion application that uses an Oracle Database where each
application user, is also a database user. That way, each user can connect
to the database through other tools beside the application, like TOAD,
SQL*Plus, etc.

For security reasons I would like to prevent these users to connect to the
database using any other program than the application itself(No SQL*Plus, no
Toad, etc …). I thought in using a "trigger after logon" with a kill session
statement, but it's true :) a session can't kill itself (ORA-00027)

At the moment I'm not figuring out a way  to achieve what I want. Can anyone
give me a hand ?

Thanks in advance,

Ricardo Bateiras.

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