If they tell me they don't like something in front of the others, my reply is "you don't need to like it: you do need to eat it!"
That usually stops the imitators in their tracks, and sometimes the fussy eater will have a go, too.
It's a big problem. I'm sensitive to real dislikes and accommodate them so long as it's not restricting their diets. Same with allergies. Sometimes strong dislikes are linked to intolerances (but not always!).
Some of the strategies I've used are:
Likes/dislikes sheet - they can dislike a food, but not a category, for example I'll accept "bananas" but not "fruit"
Tasting tests - I had a child who claimed to hate cheese, so I bought a selection of different ones and they had a sheet to mark as "Yum" or "Yuck". To my amazement, this child happily marked every cheese as Yum!, including stilton and gorgonzola, but marked plain cheddar as Yuck!
I've tried similar with other foods with some equally startling results.
Menu Preparation - the kids choose the menu and I cook it for them. We designed a proper restaurant menu, often with a theme (Italian Restaurant is popular).
Cooking - getting them to cook a simple meal. I find they're more likely to eat it.
Pasta - I make a wide range of pasta sauces that might not be absolutely authentic 
. They love pesto, so I sometimes make it with cabbage and basil, or a tomato-based sauce will have aubergine in it. Sometimes just presenting the food in different ways does the trick.
Every child claimed they hated mushrooms, so I gave them mushroom ravioli, which they loved - then I told them what it was
If I'm cooking a casserole, I always put the cabbage, kale, spinach whatever on top of the casserole to cook at the end. That way, even if they won't eat it, the juices have gone into the casserole. I use the juices from vegetable water for gravy for the same reason.
Having said all that, whatever effort you may make, keep it all low-key to the child. If they choose not to eat, then fine - up to them - you assume they're not hungry. They don't get anything further, and no attention. Try smaller portions - sometimes they feel a bit overwhelmed by seeing all that food and they can always ask for more.
Nip any disparaging comments in the bud and come down on rudeness like a ton of bricks. I once had a 7 year old boy telling me my meal tasted of dog-sh*t. DH dealt with that immediately - the boy was removed from the room and DH read the riot act before sending him back in to apologise to me and the other children. Never occurred again 