How is it possible to delete irrelevant post or reply to one who'm I talking to?
Not sure what you post or you reply to in a particular thread.
However, from your question, people can guess that your post has been deleted based on the rules below: (off-topic) no need / outside the arena.
Link:
Topic: Do not post off-topic repliesOff-topic replies have become a big problem. From now on, I will be deleting such replies without notice, forum-wide. If your replies disappear, this is probably why.
All replies must respond to the topic's original post in some way, even if they are replying to an on-topic reply. If your reply has nothing to do with the topic post, either add some content that is relevant or create a new topic.
Please report off-topic posts as soon as you see them.
If you have problems posting deleted, for the future you should fix yourself whether your reply is in the arena or outside the conversation of others.