I hope this works out well for you and for the dogs. Hopefully the owner will start taking better care of the dogs and/or the pepper spray will work. I'd personally try the pepper spray he bought, and if that doesn't work, the next time I'd try the bear spray. Maybe if his dogs come back with the spray in his eyes, he'll think about chaining the dogs up. But while I hate to say it, at some point it doesn't stop, you need to do what you need to do.

Another idea is to get some firecrackers although I'm not sure how you'd light them while riding. I through a pack of them at a dog along the railroad tracks I used to walk to high school on when I was younger. After doing that twice, the dog never bothered me again. Of course, the dogs you are talking about seem much worse than mine was, and there are many more of them.