I'm usually not interested in spending 8 mana to get 2 glorified Dazes. Why would I bother focusing on stopping it? If you spam it, that takes up most of your channeling, and I can just switch from hitting you in the face to summoning more creatures. Assuming that I was focused on hitting you in the face with my mage in the first place. I don't see how Hurl Boulder has anything to do with stopping Force Bash. It can be nice if one or both of those creatures were guarding, but it's expensive enough that I probably still wouldn't bother for any mage not trained in it.
Just to make sure, are you playing Slam correctly? It acts like a Stun until the START of the creature's action phase. So as soon as the creature is activated, they lose the Slam, and they can still act. During the creature's action phase, they're only Dazed.