create or replace procedure run_ddl (p_ddl in varchar2)
as
begin
  execute immediate p_ddl;
end;
/

SQL> create or replace trigger restrict_system
  2  before create on DATABASE
  3  when (ora_dict_obj_owner='SYSTEM' and user !='SYSTEM')
  4  begin
  5  raise_application_error(-20001,'You are not authorised!');
  6  end;
  7  /

Trigger created.

SQL> show user
USER is "SYSTEM"

SQL> create table system.t(x int);

Table created.

SQL> drop table t;

Table dropped.

SQL> connect scott/tiger
Connected.
SQL> select * from user_sys_privs;

USERNAME                       PRIVILEGE                                ADM
------------------------------ ---------------------------------------- ---
SCOTT                          CREATE ANY TABLE                         NO
SCOTT                          UNLIMITED TABLESPAC

SQL> create table system.t(x int);
create table system.t(x int)
*
ERROR at line 1:
ORA-00604: error occurred at recursive SQL level 1
ORA-20001: You are not authorised!
ORA-06512: at line 2

SQL> create table scott.t2(x int);

Table created.

SQL> create table thiru.tc(x int);

Table created.