There's blame on both sides here. You didn't ask before putting the sphere in place. Cube neighbour didn't ask for it to be removed - he put the cube in and waited for you to react. Communication is the most important thing when neighbours have issues, so neither side was a good neighbour in their handling of it.
But looking to place the blame means it'll escalate. Someone needs to back down and say sorry. Given that by default, someone's airspace is theirs unless they agree otherwise... cube neighbour is the one who gets to decide if the sphere stays or goes.
You could let it happen the hard way by making him AR you, but it wouldn't hurt to show you'd like to smooth things over. If I were you, I'd remove it and apologise. Just a straight and simple, "I'm sorry. I've taken the sphere down." Don't slip in any insults or "sorry, but..."