i agree that simply saying "no thank you" or "i'm not hungry" is the way to go. i will strongly disagree with saying that you're watching your whatever it happens to be. when you imply that someone else's food isn't up to your par people tend to insist that it is [ie want to make you eat it because if you have special limits then they want their food to fit in those special limits] and it makes a big deal about it. whereas there isn't much one can do about someone else not being hungry.
don't feel bad that this is difficult right now. learning anything new can be a little headache inducing. so accept that right now you need to work on it and focus on it and as you learn how to balance what you eat you can teach yourself tips and tricks to make it easier. but you have to learn it the hard way first. just like in math class when you learn to divide by hand before you get to use the calculator for it.
one of the best things that you can do to get your parents to stop shoving the ice cream at you is to get some healthy, higher cal options in your rotation. this article www.sparkpeople.com/resource/nutrition_art
has some great tips. but you should also look at the foods you eat regularly and try to figure out ways that you can boost the calorie count by 50 cals [or however many calories you need to to hit your goals]. it might be adding half an ounce of nuts to your favorite salad, switching from nonfat dairy to 2% or whole, adding 1/4 of an avocado to your sandwich, eating a 25% larger portion than you were before, drizzling a little olive oil and herbs on your steamed veggies and so forth. when you are already hitting your calorie goals there won't be any reason to suggest ice cream to get you there.