Can a protection spell help if someone already has voodoo on them?

In short, "maybe."

While many other religions do not, or cannot, discern between the two religions there is a large difference in the two. Voodoo (or Vodun) is inherently Christian in root, while the Wicca faith has little to do with Christianity. Additionally Voodoo is a spirit based religion, while Wicca is based on polytheism and the cycle of life.

In direct answer to your question there are many rites and rituals in Wicca targeted at stopping or preventing an external influence on the individual practitioners life. This is especially true for negative energies, those often associated with Voodoo hexes and curses. However the amount of protection provided is determined by the individuals belief and strength of faith.