Hi, I may have a few suggestions for you.

I am not too sure how big your dog is, but have you tried getting a choker collar and putting it on your dog? When ever he/she makes a mess in the house correct the dog with the collor and tell it no. This will not harm your dog. Just make sure if you do not know how to put it on the right way that you ask someone who knows how to do it like someone in a pet store or a vet. And you might want to ask about the size because you do not want the collar too tight or too loose.
I have a seeing eye dog and she has one of these same collars. Most of the guide dog schools use them and I do not think they would recomend something that would harm a service animal.

If your dog does something good like does it's business outside tell he/she is a good dog and give it lots of praise. Maybe give it some kind of treat.

Ripplegirl99