How about a voice in the wilderness?
What's idiom in English to express, the following idea:
"The person who is strictly protesting, opposing to some kind of issue or idea, has been disregarded at utmost by the community"
Your helps will be appreciated,
thanks for helping, but that doesn't involve any insulting to the opposing person,
he/she should be treated with utmost disregard and humuliation, neglecting the fact, that whteher his/her idea is useful or not.
I'd suggest the word crackpot
People such at the OP described are not necessarily crackpots, people with crazy ideas.
Perhaps crank is better. If it's an insulting word or term for someone with strong opinions in disagreement with everyone else, then it's going to have a connotation of craziness. Both crank and crackpot suggest that it's someone with strong ideas, as opposed to simply being mad.
I think words borrowed from religious writings might be apt: heretic, zealot, Judas etc.
Other words that I can think of which might help to form the specific idiom that you are looking for include: black sheep, fanatic, radical.
So, this heretic would be putting the cat among pigeons and rocking the boat with his radical ideas.
"putting the cat among [the] pigeons" must be BrE. I've never heard this idiom before.