We don't have any unsightly neigbors, but regardless, we started the long-term plan of planting evergreens to hide any view of our back neighbors. I love my privacy, and I like the illusion that there are only woods behind our home. Every Mother's Day I ask my husband to buy a tree as my gift.
Since these people have been this way for a long time, and will not likely change, you might want to start-a-planting. I know it won't help for your party, but for the future, it is IMO the prettiest way to hide what you don't want to see.
And, I get to say I am Green!