A flat poly sheet surface (quads) with the spline being the border. Extrude would produce a many sided NGon, not too difficult to clean up in this example though. Surface modifier. I couldn't get this to work before, untill I realised that the spline had to be broken down into 3 or 4 sided shapes. For some reason, the…
You may have to be a little more specific as to what shape you are trying to get. Are you trying to have a shape follow that spline? If so, you can turn on "Render In Viewport" under the spline options in the command panel. That will give you either a cylindrical or rectangular mesh in the shape of the spline. If you want…