tst> connect system/manager@tst
Connected.
tst> create trigger new_u1_proc1 before create on database
  2  when (ora_dict_obj_owner='U1' and
  3        ora_dict_obj_name='PROC1' and
  4        ora_dict_obj_type='PROCEDURE')
  5  begin if user != 'U1' then
  6           raise_application_error(-20001, 'Bad username ('||user||')!!!');
  7        end if;
  8  end;
  9  /

Trigger created.

tst> create procedure U1.proc1 as
  2  begin null;
  3  end;
  4  /
create procedure U1.proc1 as
*
ERROR at line 1:
ORA-00604: error occurred at recursive SQL level 1
ORA-20001: Bad username (SYSTEM)!!!
ORA-06512: at line 2


tst> create package U1.proc1 as
  2  a number;
  3  end;
  4  /

Package created.

tst> drop package u1.proc1;

Package dropped.

tst> connect u1/u1@tst
Connected.
tst> create procedure U1.proc1 as
  2  begin null;
  3  end;
  4  /

Procedure created.

tst> drop procedure U1.proc1;

Procedure dropped.