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.
...but threads I have visited are no longer turning purple on the thread lists, at least not on this PC. The one I use at work is still okay. What have I touched that I shouldn't have (I've looked at customise but that hasn't helped!)