From the layout window, right click on an empty point in the room. From the context menu that appears, select the "Delete" command, which will allow you to remove the room from the project.
When you delete a room, you will be asked if you would like to delete the items contained within the room, as well (furnishing items, 3D objects, groups, lights, quotes, text...).
If there are other rooms present in the project (a multi-room floor plan), any walls of the deleted room that are shared with these other rooms will not be deleted, however, their width will be halved.
This article is valid for the 2019 version and later.