I like that sign idea. I'd make several.
You could also invest in some pepper. Not spray, but ground. Certainly keeps my dogs noses out of places I don't want them, IE around the Christmas tree. Pour that along the spot that they normally come in... dobie gets a snootful of that then he might decide to just keep walking. Then again, with her encouraging her dogs to trespass (via throwing things and telling them to fetch) they might not... but it'd be worth a shot to me.
I'd also talk to animal control. Mention that there IS this situation. Considering her reply she may feel the same about letter her dobie off the leash so he can 'play' with people's kitties and doggies too. I'd make sure that your neighbors are aware of this threat. That way if anyone's pets turn up hurt they know where to look first. Also ask if there's any way that a truck could cruise by, assuming your animals are all legal of course, during her 'typical' walk time. If the guy sees the behavior for himself he should say something, though wouldn't be awesome if she got a ticket for breaking a leash law (is there one there?)
Seriously... would you lock a cat in a birdcage with a canary? So far beyond dumb.