Altium castellated cut outs
Instead of having two holes (one round and one rectangular?) you can create one plated slot.  That would get rid of the internal plated wall pointed to by the red DOWN pointing arrow.  Design it such that the end that you don't want fully hangs over the edge of the board.  When the fab house routes the PCB, they will cut out any unwanted plating.  You just need to call out routing for depanelization on the edges that have this feature to ensure you get what you want.