Two days ago, when I was having a conversatoin with a user in my question, there came a troll which infuriated and I tried to block him on that question but I accidentally blocked the person which I was having a conversation.
Both of us posted anonymously, which means I could not reply him on his opinion, thus I could not even find another way to contact him.
"If you want to block an Anonymous... They will no longer be able to interact with you on that post, but they will not be blocked on any other post and they won't be in your Blocked list." From GAG's FAQ So you could always repost a similar subject and hope your anonymous replies.