SQL> create or replace trigger trg before ddl on database
  2  begin
  3    if ora_sysevent='ALTER' and ora_dict_obj_owner='SCOTT' and ora_dict_obj_name='EMP' then
  4      raise_application_error(
  5        -20000, 
  6        ora_sysevent||' is not allowed on '||
  7          ora_dict_obj_owner||'.'||ora_dict_obj_name||' table  <====='
  8      );
  9    end if;
 10  end;
 11  /

Trigger created.

SQL> conn test/test
Connected.
TEST> alter table scott.emp modify empno number(4);
alter table scott.emp modify empno number(4)
*
ERROR at line 1:
ORA-00604: error occurred at recursive SQL level 1
ORA-20000: ALTER is not allowed on SCOTT.EMP table  <=====
ORA-06512: at line 3