1 Click on the magic "Manage your LearnWorld" button at the top of the screen

2 At the Admin Menu at the left of you screen click on the Settings Button and then at the "Community" option. Alternately, you can navigate to the Community first and then click on the "Edit" button. In both cases the outcome will be the same: you will find yourself in the community, in authoring mode. 

3 Now choose the message you want to delete and click on the red bin button to the right of the post.

4 LearnWorlds will ask you to confirm that you want to delete the post; click delete to confirm the message deletion.

And that's how you do it. :)