Trying to discipline an 18 month old - am I wasting my time?

[Playitagainsam]

My DD is 18 months and generally is a pretty chilled out little girl (so far!). Quite sensitive and aware of what's going on around her. I rarely have to tell her off but for one thing - she always throws food from her highchair. She's always been quite a fussy eater and for as long as I can remember she's been chucking bits of food or her drinks on the floor. At first I tried ignoring it, she carried on. Then I tried telling her no with a stern face/firm voice and it's made no difference. If anything she seems to enjoy it a bit more! I am guessing this is pretty standard toddler behaviour, and I entirely know that in the grand scheme of things this is not a big problem. But it drives me round the bend!
So my question is, am I completely wasting my time even trying to stop her? Do they have any comprehension of discipline at this age, or do they basically not give a stuff?
Would welcome any advice from more experienced parents and apologies for the trivial nature of the post!

With our DD we take the food/drink away immediately if she throws, drops or spills it deliberately. When younger she'd have had one or two warnings.
I am aware she's never been a fussy eater and only started throwing/dropping food after she'd eaten her fill. We might have done it differently otherwise.

Personally I don't think ignoring really helps. I find with my dd that she responds best to being shown an alternative. When dd was going through her food throwing phase we would tell her if she didn't want it she could leave it beside her plate and if she had finished she could push her plate away. If she did throw something it would be returned once but after that it stayed on the floor, because that's what happens if you throw stuff away. She never throws food now and will just push her plate away if she doesn't want it.

I do similar for other issues like scratching, I would certainly never ignore it if dd was hurting another child. She went through a hair pulling stage at one point. I dealt with it by removing her from the other kids if it was a child's hair and telling her pulling hair hurts (an exaggerated yelp of pain always helpedhelped when oit was my hair) but stroking was nice. I would then take her hand and show her how to stroke hair gently. It took a few weeks but got feedback from nursery that dd had been going up to other children and stroking their hair :o

For dd I have found showing her an alternative is pretty much essential if I want her to stop doing something.

Thanks everyone for the ideas, it's much appreciated. You're absolutely right, she generally starts chucking food once she doesn't want anymore, or she doesn't want what I'm giving her. So this morning I only have her two chances then it was out of the chair and the rest went in the bin. Only problem being sometimes she's hardly eaten anything - but then I know the old theory that they won't starve themselves! I also try just giving her one or two bits of food at a time, but as she's really not mastered cutlery yet, sometimes it can feel like we're regressing back to the baby feeding days!
I've tried so many times to ask her to put food on her table rather than drop it, she will only occasionally do this. But then she will sometimes try to put it in my hand instead, so maybe I just need to keep telling her no to dropping it but to accept her passing it to me instead.
Thanks again everyone for all the thoughts and suggestions.

Haven't read the responses but what I do is give him an empty bowl/pot and encourage him to put the food in there and most of the time it works! He even tips it back out onto the tray to try again. He's 19 mo.

It's the gravity game! If you drop it, it falls!

And again!

And again!

She's doing (very, very) basic science. Give her things to do it with that aren't food while you feed her.