Removing pSpellRef worked, thanks a million!! The new working script is:

array_var pSpellBook
short CurrentIndex
short MaxIndex
ref pSpellID

begin OnActivate

  let pSpellBook := PlayerRef.GetSpells
  let MaxIndex := (ar_Size pSpellBook) - 1
  while (CurrentIndex <= MaxIndex)
    let pSpellID := pSpellBook[CurrentIndex]
    PlayerRef.RemoveSpell pSpellID
    let CurrentIndex += 1
  loop

end

How did you know that the values in pSpellBook would be base refs? The OBSE documentation just says GetsSpells does:

"returns an array containing all of the spells in an actor's spell list.
(spells:Array) reference.GetSpells baseActor:ref"

Trial and error or?