Oracle SqlPlus - saving output in a file but don't show on screen

Right from the SQL*Plus manual
http://download.oracle.com/docs/cd/B19306_01/server.102/b14357/ch8.htm#sthref1597

SET TERMOUT

SET TERMOUT OFF suppresses the display so that you can spool output from a script without seeing it on the screen.

If both spooling to file and writing to terminal are not required, use SET TERMOUT OFF in >SQL scripts to disable terminal output.

SET TERMOUT is not supported in iSQL*Plus

Try this:

SET TERMOUT OFF; 
spool M:\Documents\test;
select * from employees;
/
spool off;

Try This:

sqlplus -s ${ORA_CONN_STR} <<EOF >/dev/null