Your take on spell flexibility

To what extent do you allow using spells for completely unintended purposes?

Example that comes to mind: Ice knife.

The spell says "You create a shard of ice and fling it at one creature within range"

What would be your opinion if your player said "but I choose not to throw it"? Can he keep a shard of ice to knaw on as a snack, or perhaps as a generic sharp object (albeit subject to melting)