Do you need a topic for one-way communication for announcement purposes? Here's how you set your topic as 'View Only'.
1. First, create a topic if you don't already have one.
2. Go to the upper left corner of the topic and click on the [V] button and select [Edit info].
3. Select [Set view-only] check box.
This way, only admin-level members can upload files and write messages in the topic. But please keep in mind that members are still able to comment on files.