tt01...you're NOT the only one.
They just don't listen sometimes. A friend of mine has a son who is seriously allergic to strawberries. Fortunately, they aren't fatal to him, but they make him feel wretched. You would think this would be a good enough reason to refuse food.
But alas, my friend was at a dinner party where they had some sort of strawberry dessert and her little boy politely refused. The auntie grabbed a spoonful of the dessert and shoved it in his mouth. He threw up all over her new rug that she'd had flown in from overseas.
Served her right.
I'm beginning to suspect that you simply have to say no politely and be firm. If they get offended, well...I guess you have to shrug and move on.