Okay, here comes the cruelty to animals hit for the week:

For persistently problem animals (be they dogs, cats, racoons, bears, even flies), nothing is more effective at neutralizing them than a good laser hit on their retinas. You can get laser pointers now that have enough power to instantly do permanent damage to any animal's vision. I've even immobilized rabbits in gardens with them, to the point I could walk right up to them and grab them by the ears.

If you want to permanently solve a nuisance dog problem without leaving behind any trace of the attack, I can't think of a better solution than that. It might be more expensive than pepper spray, but you'll only have to do it once per animal.

Okay, bring on the flames folks.