You need to change the curb approach property of the individual stops to say "No U-Turn".
You can do it for all of them by opening the Stops Attribute table and then using the field calculator to set the curb approach to the value 3. And then re-solve. You can read more about curb approach here:
http://help.arcgis.com/en/arcgisdesktop/10.0/help/index.html#/Route_analysis/004700000045000000/
Jay Sandhu