Get another Rat Terrier.

We have had many dogs, and the RTs were the only ones that could actually CATCH and kill the squirrels, not just chase them. Not only squirrels, but I had one who killed over 30 mice in under two minutes when we removed some trailer skirting in the winter.
Of course, we also had one that ate our koi fish, taking one bite out of each tasty tummy, then leaving the carcasses lined up neatly on the patio. Same dog that, after catching a squirrel and dispatching it with one bite, dropped it and stood there looking at it, wagging her tail as if to say "Tag, you're it!"
Carolyn