If you have previously recommended this thread, you should see a tick / check mark on the recommend button. Click the tick to undo the recommendation (the tick may appear to change to a cross as you do this.) If you added a comment with your recommendation, you will need to delete that from your facebook wall separately.
If you go into your message board options, there's an option to bookmark threads. You can then bookmark the last message you've seen in a topic and if, in future, you click on the thread title from any of the thread lists, it skips to that message.