Discussions

We have a one year old dog that we let into our fenced back yard when we are home. Our neighbor always lets their cat out free to roam. We had problems this winter with it peeing on our deck. Tried to talk to them, but they kept letting it out.  
The other day I heard my dog barking and went outside, and she had the cat cornered in the yard. She went after it, just protecting her space as its natural for her to do. I ran to the corner to open the gate to let the cat out before my dog really did some damage to the cat. Neighbor heard the commotion and came out yelling at me, for letting my dog go after their cat. It was in my yard!!! I told him that next time I might not be able to come in time to let his cat out.  
Yet the next day, there is the cat again in my yard.  
Don't know what to do to get it through to the neighbors that it isn't safe for them to let their cat out and roam free! I like cats and I would feel bad if my dog killed it, but don't see what I can do! I don't think it's fair for me to have to watch my dog all the time to make sure she doesnt go after the cat, she is very well behaved on her own outside  
Any suggestions on what to do with these neighbors?