Oh, gosh, Tammy, you really need to confront those people directly since they are using your yard as a bathroom!
What nerve!
I know you hate to talk to them, but if you don't, you will only get madder . If you hold it in, you're likely to explode one day and say things you regret afterwards. Your relationship will really suffer then.
It just amazes me how inconsiderate and stupid people can be with their dogs.