Can I freeze the Q&A?

No, you can't freeze the Q&A. But you can use the question limit feature to stop incoming questions.

How? Set the question limit for your buyer group(s) to 0.

When the limit is set to 0, Question Submitters won't be able to ask new questions, but are still able to see all submitted questions and disclosed answers.

Q&A Admins are still able to answer submitted questions when the question limit is reached.