they have hung up their lights in our tree!
OK, I do admit that our tree, in the corner of our front garden, does overhang their garden.
And they have put up their lights in the overhanging branches only ... but BLUE FLASHING LIGHTS?? In OUR TREE??? People might think they are our lights 
So AIBU if I sneak in the middle of the the night tonight with a pair of scissors.... Mrs Stubborn is encouraging me, but i am not quite sure...