Can I still stop using GPS while on a phone call?

It does. If you want to interfere with A-GPS but still want to talk on your phone, you need to use a GPS jammer. This device blocks all GPS signals in its working area, so the A-GPS module on your Sprint CDMA phone will not work. Even if the A-GPS module starts working, it is completely useless without a GPS signal to find your location, A-GPS will only speed up the process.
As for the phone, I would recommend that you keep using your CDMA phone, as GSM phones usually have an A-GPS module in one with a GSM module. So if the work of A-GPS is interrupted, there may be communication problems. CDMA phones don't have this problem. If you make your A-GPS module unusable without GPS signal, you can make calls, write text messages, etc.