I understand how people having you in their circle means that they can send requests due to how G+ itself is set up- though, I am not sure why the G+ account setting to block notifications from people outside my circles doesn't work for this as well.
But the huge issue, imo, is that they can and most certainly do still barrage me with gads of requests every day even though I have blocked them on Google+. While people you block on G+ can still see any public posts you make, I haven't had someone blocked be able to send me notifications in any other facet of G+.
With no way to stop the ever-increasing spam/ intrusion, the neighbour/ gifting functionality is completely broken for me. I can't wade through it, and this is by far the most off-putting bug I have seen since I started playing last fall (way worse than no neighbour help at all imo)