It isn't possible to use just the left bud of the SoundSport Frees, because of how they are designed to connect to your devices. The way they do this is the signal goes from Device > Right bud > Left bud. The left earbud doesn't connect directly to your Bluetooth device, only the right bud.
With this signal path, it is possible to use the right bud on it's on, but not the left.
I hope this helps! Let me know if you have any further questions!