I'm mad at my dogsitter for their dog attacking mine
Yikes! How did they respond to the attack?
They're not taking responsibility. The person I talked to on the phone didn't even seem to believe me. I'm very angry.
They were wrong not to take responsibility for their pet. You should report them so that others know they are not responsible pet sitters.
I'm going to tell my Facebook friends and castrate their dog.
Uh, telling your Facebook friends is sufficient. Abusing animals will not help your case. You will seem like the better person if you respond appropriately.