Asking if you can tie the dog when you take yours out sounds entirely appropriate, and I can't imagine the owner being offended. As to spraying the dog, the product that would be effective and what I sometimes carry on walks is citronella spray (brand name "Spray Shield", I think), which is discouraging without being harmful to the dog in any way (won't discourage a serious attack, but will back off a curious or overly friendly dog). Bonus that the owners tend to get aggravated that you sprayed their dog with something, and they keep it away from you next time. It's +P for dog owners, really.
I wouldn't use that in this case, though, because the owners might be offended by it, even though it's harmless. The private property thing.