does he have his own laptop. if so, he has root.
If you believe this is bullet proof (it is not, especially if he doesn't follow rules in the first place), go for it.
but - very easy to workaround.
I would drop the "please" from the error message ;)
and bear in mind also that if DSSADM is a DBA, this will not work, failed logon triggers do not affect them
ops$tkyte%ORA10GR2> CREATE OR REPLACE TRIGGER SESSION_ACCESS AFTER
2 LOGON ON "OPS$TKYTE".SCHEMA DECLARE
3 v_username VARCHAR2 (30);
4 v_osuser VARCHAR2 (30);
5 BEGIN
6 raise_application_error (-20001, 'Please use your own account to login');
7 END;
8 /
Trigger created.
ops$tkyte%ORA10GR2> connect /
Connected.
ops$tkyte%ORA10GR2> !tail /home/ora10gr2/rdbms/log/alert_ora10gr2.log
Thu Apr 2 17:09:28 2009
Errors in file /home/ora10gr2/rdbms/log/ora10gr2_ora_25592.trc:
ORA-00604: error occurred at recursive SQL level 1
ORA-20001: Please use your own account to login
ORA-06512: at line 5
Thu Apr 2 17:09:35 2009
Errors in file /home/ora10gr2/rdbms/log/ora10gr2_ora_25593.trc:
ORA-00604: error occurred at recursive SQL level 1
ORA-20001: Please use your own account to login
ORA-06512: at line 5
... If i were to login as dssadm through sql developer ....
and if I were on your system, it would work, because I'd just create an OS user account.... on my own machine.