May we stop a neighbour building an undesirable dividing fence? Need a stick to go with quiet friendly voice.

It may fall under the "spite" fence issue, there are legal limits on how high you can build a fence, most places is up to 8 ft, above that, your doing it for no good, legitimate reason except to show contempt and 'spite". for your Neighbors. check it out with your city ordnances, there is usually some type of city code about size and types of fences in every place people live.