Yellowing spikes are absolutely 100% normal and have no effect whatsoever on the plant's health. Some Phals lose their spikes soon after the blooms fall off, others keep bare green stems for a long time. Also it's not yellowing stems that you cut back to a node (since they are dying anyway), it's green stems. There is no way to stop the process, once a spike starts to yellow it usually does so all the way.
The roots look healthy enough (hard to tell from a small photo), so your plant is probably dehydrated from lack of water. Repot into fresh medium, and try increasing your watering frequency a bit. In any case, even if your plant does have a problem and is dying, it is absolutely not because of the yellowing spike!