I can see this escalating quite badly for all parties.
Is there no way you can all have a grown up chat?
They aren't the type to chat and the current arrangement of them parking outside my house is the most optimal for them. They will only stand to lose.
My brother has parked outside their house before and they tried to have a go at him. When I heard about that I had a stern word with their family. I wouldn't mind reaching a harmonious solution but it won't happen.
Sounds a lot of chew to pee off a neighbour. Technically it is street parking so they're not your spots. Why not just park over their house to make a point?
Send them all glitter bonbs
Given they have 4 cars, they tend not to leave the space outside of their house free and will move cars into place quickly. It's rare that I see a spot to steal.
Of course, the parking spots don't belong to us, which is why I have been keen to say 'space outside my house' rather than 'my spot' but it is selfish of them to immediately nab the area I would park in and force me onto the grass. I would be more understanding if their cars were worth anything but the banger I am looking to buy as a parking space reserver would probably still overshadow theirs.
--
SORN wouldn't work because it has to be off the road.