John:
The forum software has a few options and limitations.
Sometimes we can remove an offending reply quickly enough that it has no 'children', thereby preserving the original thread.
Sometimes we can split a thread and delete the part that offends, preserving the original thread.
Sometimes we can simply lock the thread and block the offending member.
And sometimes we can ban the offending member (if the infraction warrants it).
But if we delete a post or reply that has children, the children disappear as well. A lot depends on how the reply is made. Sometimes the person who replies clicks Reply at the bottom of the original post. Sometimes they click Reply at the bottom of a specific other Reply. If no one subsequently Replies to that Reply, then it has no children and can be safely removed.
Each of the Moderators has his own "style". I tend to lock threads if they have otherwise useful information. Sometimes rather than deleting a "bad" reply I edit it to remove the text, then lock it. For me, if the thread has useful info in it, I try as hard as I can to leave it intact except for the offending reply.
A lot also depends on how quickly a Moderator notices the offense. If we catch it earlier, there's a better chance we can leave the original thread intact. But we're not always here continuously ... sometimes things fall through the cracks.