Notgran Don't go. Almost all posts are genuine, it is only a few which are flying a kite to see what reactions they get.
You can always tell if it is someone who has only joined this minute to post rubbish. If you put their name into the search box at the top of the page, it will list posts made by that username, or ones mentioning them, up to the previous day. If there are none, then it is possible that poster is new. Of course, they could have joined long ago and until now have been reading without posting, but it does give you a clue.
The best thing to do if you are not sure is to report the post. Gransnet (or Mumsnet) will take a look and decide. They don't mind doing this - it is better than ranting at an innocent poster in reply to an ambiguous post.