We had tried everything. We drilled holes in our wall and shot in expanding foam, we poked the squirrels, we sprayed them with ammonia. No avail. Squirrels don't eat poison (it is illegal to kill them in NYS), and they are persistent.
The exterminator actually ended up working from the inside. He cut a big hole and then filled it with rouble, wire mesh and expanding foam.
This time, we know this worked as the squirrels sat outside out window on the scaffolding looking at us for three days. They were trying to figure out how to get back in. Thankfully, this time we finally foiled them, and we are finally squirrel-less!
Hudson still says "shoo-shoo sqwuel" any time he sees one on the street. Smart kid.