With phals, my general practice is to leave it alone. If it wants to rebloom, it will. Cutting the spike to force it will definitely weaken a plant, as Ross states. If the tip starts to yellow, I let the plant resorb the nutrients, and when it's brown and brittle, off it comes, which is how I treat all others.
To the best of my knowledge, no oncidium reblooms from the same spike. Psychopsis do, though.