[Your Name, & Address here]
[Recipient Name & address here]
I really appreciate your privacy, and I have no problem with whatever you do in your house; however your pet Dog creates problem for me these days.
Bruno, your dog poops in my yard every other day. And, it is really an annoyance and I cannot take it anymore. So, I would really appreciate if you will not let your pet enter my yard from now onwards. I believe it is your responsibility to make sure that your pet does not create any kind of problem for your neighbors.
And, if this problem persists, then I am definitely going to ask you to come over and clean my yard immediately.
Thank you for your understanding, and fast action regarding this problem.