How to train a Pit Bull mix off cats?

I basicly own a American Stafordshire Terrier mix with a Assie Cattle Dog and maybe some collie. Mostly APBT/AST. How can I train her to not try to kill our cats? She ignores the Prong collar when I use that, but she seems to come most of the time when I give her the command. Just not with cats. Please help me!