There is currently no way to do this in the UI of the tool. However, the schema elements for generating different update and delete actions are part of the generators and should generate could correct SQL if they are set, but there is currently no way to direct the generator to make this change.
It should be possible to add markup similar to the custom-properties based solution for adding schema names shown in http://www.ormfoundation.org/forums/p/573/1706.aspx#1706. The main difference would be that tracing a reference constraint back to a specific role that could hold the custom property. Due to absorption and the possibility of multiple steps in ORM space corresponding to a single reference at the relational level, the traceback from a generated reference constraint to a role is not quite as trivial as matching a generated table to an object type. However, I should be able to get something that works for the majority of cases.
Give me a few days and I'll try to fit in the xslt exercise to enable a custom property that will control this generated SQL directive. In the meantime, if you want to change the overall default, look for 'onUpdate=' in C:\Program Files\Common Files\Neumont\DIL\Transforms\DCILtoDDIL.xslt and change the default for onUpdate (and onDelete) to one of the values in the referentialAction enum in C:\Program Files\Common Files\Neumont\DIL\Schemas\DDIL.xsd. The xslt changes should apply immediately if you re-run the custom tool ('Run Custom Tool' is on the context menu of the .orm file in the Solution Explorer window).
-Matt