Follow below steps for disabling auto tasks in oracle database.

SQL>  select client_name,status from dba_autotask_client;

CLIENT_NAME                                                      STATUS
---------------------------------------------------------------- --------
auto space advisor                                               ENABLED
auto optimizer stats collection                                  ENABLED
sql tuning advisor                                               ENABLED




set lines 180 pages 1000
col client_name for a40
col attributes for a60
select client_name, status,attributes,service_name from dba_autotask_client
/

-- DISABLING SPACE ADVISOR


BEGIN
  DBMS_AUTO_TASK_ADMIN.disable(
    client_name => 'auto space advisor',
    operation   => NULL,
    window_name => NULL);
END;
/


-- DISABLING AUTO SQL TUNING ADVISOR


BEGIN
  DBMS_AUTO_TASK_ADMIN.disable(
   client_name => 'sql tuning advisor',
    operation   => NULL,
    window_name => NULL);
END;
/

-- DISABLING AUTO OPTIMIZER STATS

BEGIN
  DBMS_AUTO_TASK_ADMIN.disable(
    client_name => 'auto optimizer stats collection',
    operation   => NULL,
    window_name => NULL);
END;
/


SQL>
SQL>  select client_name,status from dba_autotask_client;

CLIENT_NAME                                                      STATUS
---------------------------------------------------------------- --------
auto space advisor                                               DISABLED
auto optimizer stats collection                                  DISABLED
sql tuning advisor                                               DISABLED