how to delete a DATABASE from command prompt ?

[stecal]

Assuming all your datafiles are in /opt2/oradata/baddb -

as the owner of the files (oracle, oracle8, oracle9, whatever), do
rm /opt2/oradata/baddb/*

That will remove all files in the baddb directory.

If you are deleting the database (not re-using "baddb" anymore) -
rm -r /opt2/oradata/baddb

That will not only remove the files under the baddb directory, but will also remove the baddb directory.

Don't forget the control files (if you left them in $ORACLE_HOME/dbs).

Check system processes for anything to do with baddb -
ps -ef | grep baddb

Kill any remaining processes with
kill -9 (the process id you got from the ps command)

But, it sounds like you may be playing with fire by the nature of the question.