The policy of Stack Exchange sites is Anybody can ask a question and Anybody can answer. This is displayed when someone joins a community.
Now, What should we do if the OP of the question ask certain user(s) not to post an answer (because he already know the opinions of the user or in the worst case due to opinion clashes)?
For example they added a note at the end of the question like this:
Dear X, I have seen your opinions many times regarding this topic and I know what you will say. I suggest you not to answer this.
How should we handle situation like this one?
This is what I have seen from a user on one of the sites I participate and such a note is removed by the OP after being asked to do so.