Hi Teresa,
Either of those options may work to remove one topic thread from showing up on search, however if you decide to uncheck the robot index option that may remove the entire site it would not be for only one topic.
Would recommend first moving the topic/thread to a member-only category, then even if it does appear in search that would most likely only be temporary until a new index refreshes search to show that is no longer visible to crawlers but only logged-in members.