How to enable and disable Automatic SQL tuning

Enable:

BEGIN
dbms_auto_task_admin.enable(client_name => 'sql tuning advisor', operation => NULL, window_name => NULL);
END;

Disable:

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

If desirable, Oracle also lets you enable or disable all automated maintenance tasks for all windows. Do this by calling the ENABLE or DISABLE procedure with no arguments:

EXECUTE DBMS_AUTO_TASK_ADMIN.DISABLE;

Sources:
dbms_auto_task_admin
Configuring Automated Maintenance Tasks