I would in no uncertain terms let them know the dog is not allowed on your property anymore and if he comes over you will call animal control. And any vet bills you incur if your dogs get worms and or fleas will be delivered promptly to them for payment. They always say, fences make the best neighbors. Maybe if you cant get a regular fence righ now, you can get chicken wire or something just to keep the other dog out. It is cheap enough.