login as system
insert into SQLPLUS_PRODUCT_PROFILE values ('SQL*Plus', 'YONG', 'HOST', null, null, 'DISABLED', null, null);
create user yong identified by yong;
grant dba to yong;
conn yong/yong
SQL> host ls
SP2-0544: invalid command: host

On Windows:
SQL> host date
SP2-0544: invalid command: host
 
login as system
delete from SQLPLUS_PRODUCT_PROFILE where userid = 'YONG';
commit;
conn yong/yong
SQL> host date
Tue Jan 28 12:50:11 CST 2003



References

SQL*Plus User's Guide and Reference, E Security