Hi Andrea,
I am not sure what prasoon has told you but I am sure he may have told you that As per standard deign, system will allow to delete the PO once the procurement scenario is completed with GR and IR
I am sure that it is possible through user exit
check user exit I guess it is "EXIT_SAPMM06E_017" consult with abap consult, take a logic for po history
sap 457511 may help you.
please let me know if you have any doubt
Regards
Mohd