Nothing to prevent it but a DM's self-respect. Any player who tried that should probably told right out that the spear doesn't work quite that way in a narrative sense. Elric of Melniboné doesn't go running off into the woods crying out "Blood and souls of chipmunks for my Lord Arioch!" in order to charge up Stormbringer. The same should apply here.
Well that and the fact that you could kill 100 small woodland animals and still only have 1 THP to show for it.
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