I dont think there is any reliable way to do this.
You could watch for "user-agent" string (which the mobile device owner can change) and tcp windowssizes being used (which can also be changed).
One way is to do the other way around - use globalprotect to identify which box is which and based on this information (like a whitelist) you can QoS those into a higher prio. Or captive portal (but this wouldnt stop mobile devices).
Another way could be if you have an AD and configure userid to only allow the specific AD-group containing your approved devices (based on machine accounts).
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