Yeah one way is to switch to a view where you're viewing the curves edge-on to their plane, and then use the corner sizing grips to squish it down until you get "flat snap" engaged. There's an animated example here: http://moi3d.com/forum/index.php?webtag=MOI&msg=3378.4
You can also draw a plane surface and use Construct > Curve > Project to project curves onto that plane surface.
