Only administrators and moderators can perform these actions. Closed topics will be invisible to regular users. Moderator can only close or open topics in the category that he manages.
1. Select a topic by ticking its relevant checkbox, then click 
on the Action bar if you are in a topics list;
Or, click 
on the Action bar if you are in a topic.
2. Click Close or Open from the drop-down menu.
The icon of the closed topic will be changed to 
automatically.