I agree with Chris - I'd try an explanation, since you never know, it might work, and I tend to feel it's more respectful anyway to avoid doing coercive things (like taking something away) without warning. Only once though - if you tell her not to put it in her mouth, and she does so anyway, calmly take it away. I also tended to try to avoid giving DS the chance to misbehave in the same way repeatedly, by, e.g., not letting him have something I'd had to take away from him the previous day, unless it was something he really, really enjoyed having. It doesn't sound as though she's very into crayons yet (unsurprisingly, even for a giftie!) so I'd tend to just keep them out of the way. There are so many unpredictable things one has to say No to at this age, that I think it's best to avoid introducing avoidable situations where one has to.
If what she's really into is turning the implement over in her hands etc., rather than making marks with it, can you identify something the right kind of shape that she can safely put in her mouth? Then if she has a pen or crayon in future and you have to take it away you can give her this safe thing instead, e.g. saying "That mustn't go in your mouth, but this can."
By the way, I really recommend the Faber and Mazlish (sp?) book How to Talk so Kids will Listen and Listen so Kids will Talk - it's never too early :-)
