The member posting immediately after the deleted post runs the risk of being considered the necro-bumper of the thread.
Their posts will get deleted along with the first one (if it gets reported and dealt with). So you shouldn't really run into that problem.
My solution to this problem is to lock the thread after the post has been deleted. If the thread has been inactive for more than a year, then it probably isn't newsworthy any more. If new news does arrive, then it is a simple matter to start a new thread and to link to the old one in the first post.
Locking a thread in this scenario is a good idea.
One question though. How long does something need to be inactive for it to be considered Necro-bumping? I nearly reported one this morning that was last active in January. It was a pointless post but I wasn't sure if the thread was old enough to qualify.
Edited because I slightly misunderstood your question the first time.
