October 8, 2015
The residents of
[address]

Dear Neighbor,
This afternoon, when my daughter was in our back garden, she overheard a voice from your yard talking to a dog. She then saw dog dung being thrown over the fence into our garden. When we investigated, we found a pile of dog dung behind our hedge, one that has been accumulating for months if not years.
This is neither civilized nor decent. Stop immediately.
Sincerely,
Mme. Hardy

Note that I am striving for icy rather than threatening. Threats will be in the next (God, I hope not) letter. I did my best to trace the owners of the property so I could address them by name, but the named owner is clearly not the resident.

What my daughter actually overheard: "I swear, all you do is eat and poop."