Is there any animal control in your area, or SPCA? I know we don't really have AC, the only option for us it to truck a dog into town to the SPCA ourselves. If it is an option though it might get the owner's attention a little bit, maybe enough to secure the Rottie a little better.
Other than that you could find a way to scare that dog off. We had a rather irritating Jack Russel (belonged to some annoying neighbours) hanging around our house for a while and going at our garbage the second we put it outside before going to the dumpster. My DH used an airsoft gun, just shot in his direction (but far enough away to not hit him), but it surprised him and warned him off for a little while. Not a nice solution, but better than a possible dog fight. There's also the cattle-prod or pepper/bear spray option. Again not nice, but might keep the dog away if there is no way to convince the owner to be responsible about it.
Other than that you could find a way to scare that dog off. We had a rather irritating Jack Russel (belonged to some annoying neighbours) hanging around our house for a while and going at our garbage the second we put it outside before going to the dumpster. My DH used an airsoft gun, just shot in his direction (but far enough away to not hit him), but it surprised him and warned him off for a little while. Not a nice solution, but better than a possible dog fight. There's also the cattle-prod or pepper/bear spray option. Again not nice, but might keep the dog away if there is no way to convince the owner to be responsible about it.