It's not complicated. Your SPSV vehicle licence is only valid if you hold a SPSV insurance policy in respect of the vehicle. If your SPSV vehicle licence is not valid you are legally required to return the tamper proof licence discs to NTA. You must tax the car as a private car if it is no longer licensed as a taxi and you intend driving it on a public road, otherwise you should register the vehicle as "off the road" and refrain from driving or parking it on a public road.
TBH, it couldn't be more simpler.