Interesting question, really. I'm not knowledgeable in this specific avenue of law, but I would assume that any action not including obvious, criminal acts would be protected by law on the part of the property owner. To be an act of animal cruelty, the neighbour would have to do something like corner the animal, preventing it from getting away, and then kick it repeatedly, I would think. Again, I don't know, but I am fairly confident that the law would support a property owner who kicked a trespassing animal so long as it wasn't demonstrable that the guy intended to hurt the animal purely for the sake of hurting it - which would be an easy thing to demonstrate.
If it were a human trespasser, the property owner would be required to warn the person and demand they leave his property, and if this wasn't heeded, and the property owner felt threatened, he would be within his rights to kick the dude in an effort to defend his property. Sort of. In dealing with a dog which hasn't the ability to heed warnings nor understand property lines, a kick might be the only way to ensure the dog gets off your lawn. In this context, even if the kick broke multiple ribs, and the dog was tiny, I'm very confident the property owner would be legally protected. So long as his actions and intentions were clearly aligned with removing the dog from his property. Excessive force in executing this right? maybe, but that's gonna be a hard case to win, I'd think.