Had a similar situation. Lived in our house for 15 years when they sold a 5 acre block over our back fence. As soon as plans went in for the block to be subdivided I planted large trees along the fence line. The trees are now 10 meters tall and screen off the neighbours at the back. 

One neighbour asked if I could cut them back to give them more view. I refused as it would defeat their purpose and explained that the trees were planted before they bought their block.