Some ideas to consider:
* Delete each unacceptable (off topic?) replies manually.
* Ask your moderators to clean up the replies up for you. Assigning moderators who are online often, can help to prevent issues from getting out of hand, such as this one has.
* Move only the very few replies you want to keep, in to a new topic. Delete the old topic which contains the many undesirable replies.
* Close the problem topic. Start a new topic.
* Create a guideline in the OP, and possibly the Forum Intro, for what you expect in the replies. You may also want to list what is unacceptable. If closing the old topic, describe why it is being closed. Point users over to your new topic.