Is she on a leash when she's chasing squirrels? That's really the only way to stop this behavior, because, as you said, it is very self-rewarding. You can use a long-line leash, long rope, or strong retractable leash to give her some distance to run but still be attached to you. This way, when she sees a squirrel and freezes, make a lot of noise (don't call her), and when she turns toward you, get really excited and turn around and run away. Hopefully, as long as your timing's right, she should chase after you instead of the squirrel. When she gets to you, give her a HUGE reward.