If you have recently inserted a topic in a group, you can use 'undo'; it will reverse the last change (ignore, merge, etc) that you did and you can apply it several times to reverse several steps. If you need to remove a topic from a group, and 'undo' is not a good option there is another way, and it is easy to perform if you follow these steps:

Click The Eject Button.

Your topic is removed.

Update and Save

Thats it!