I'd love an allaround solution for this problem but as an Audio Engineer I'm pretty sure there is not much you can do that makes sense financially. The only REAL way to get anywhere near 0db (which is almost impossible) is to build an isolated room in the room. The room would be seperated from all walls and the floor (dampening feet).

All this foam stuff doesn't have too much sound absoption effects. It's usually not made to insulate the room from the outside but rather make waves in the room not jump around everywhere. This CAN help to make it quieter for the neighbours but not much. It usually helps better with things that have a volume knob as they don't have to be as loud to appear louder. This is not helping with mechanical pinball sounds though.

I guess you best bet would be an VPin. I don't like the idea but hacking your pinball machine to be quiet won't work without sacrificing gameplay in some way.