Hi Ahmed,
As per the standard system the system would allow to delete the posted request which will again for approval.
So in order to restrict it either you can go ahead with the change to the component configuration as suggested by Mr. Moshid but the delete button will not be available always in this case i.e employee cannot delete the leave request before approval or otherwise if you dont want to change this go ahead with the BADI CL_PT_ABS_REQ.
Try to include the custom logic in the BADI and check the results.
Regards,
Mithun K