I had a problem with BT and I cancelled each mandate in branch but they can simply generate new mandates and the cycle continues. Had to threaten legal action before they would stop.
Originator Plus should be banned imho, It gives the originator too much power.
So it is much as I thought, the banks have not yet bothered to add the capability to 'Block' a OP/ DD, even do their own rules allow it. The UK (BACS) version of the system works differently. The originator first has to send a 'electronic' mandate to the bank to set up permission for the DD. The Irish version just requires 'OP/' to appear in the DD’s on the narrative field.