I'm surprised that there isn't a means of moving an individual message to another category as it would be really handy and would help with all those people who insist of posting off-topic messages in a particular category/discusion.
Well messages are tied to discussions and discussions are tied to categories. Both are 1 to 1 mappings. If you moved a single message into a different category it would be in no mans land...
IMO, moving a single comment from a discussion to another makes no sense.
a discussion is a flux of comments, i.e. context, and it doesnt make sense to change the context of a comment.
a solution could be to extract the comment from the discussion and put it like in a trash or an alike container if they are considered as spam or off-topic, but not in another discussion.
Sometimes you get a comment that is better off posted in another discussion because they relate to that discussion and are totally off-topic in the one they are currently in - or they deserve a discussion of their own.
Comments
Shouldn't be a problem if you're administrator. ModTools extension might make this easier if you just need a more obvious way to do it.
a discussion is a flux of comments, i.e. context, and it doesnt make sense to change the context of a comment.
a solution could be to extract the comment from the discussion and put it like in a trash or an alike container if they are considered as spam or off-topic, but not in another discussion.