If you have started drawing a polygon and for some reason regret it you can press ESC to delete it.
To delete a polygon after it has been drawn you first have to select it with the S key. You can either press the button on the toolbar or use the S hotkey. After clicking the polygon with the left mouse click presse the X button or the X hotkey. It will then ask you to confirm.
A usual case with import of room objects/polygons is that some of the objects overlap. To workaround this you select one of the room objects Edit it with the M key. Create an protrusion and then select it with the S key and then press the X key.
Remember to press the save icon on the top right when finishing.