How do I show no left turn onto another road at and intersection?

You can add a turn restriction relation to indicate a no left turn. See more at the wiki. Note however that these restructions are not displayed on the standard OSM maps, only on specialist maps as linked from the bottom of that page.

JOSM has a turn restriction plugin that simplifies the task of creating plugins, and has built-in error checking. Although many standard maps do not display turn restrictions, they are very useful for navigation apps such as the Skobbler SmartPhone app.