Controlling which room I see:
Right-click on a room name in the chat and you can hide or display only that room BE Careful, you can filter out what you want and forget that you have done that!.
Controlling which trader I see:
Right-click on a trader’s name in the chat and you can hide or display only that room BE Careful, same warning as above!
Controlling which room I post to:
Left-click on the TO: area to the left of the chat input box to display the Room/Roster menu. Click on the room name and To: destination should change to that room. You can also enable/disable rooms from this menu.