Hi mike,

Click the extrude command and select the vertical shape.
Activate Cap ends plus Both sides and extrude to the desired lenght.

Click boolean difference and select the solid.
On the question "select objects to subtract" select the horizontal curve and right click.
Delete the unwanted shape.

I noticed that fillet was inconsistent so i uploaded a modification.
In this case i extended the horizontal curve a bid to make sure that the cut turns out clean.
It's a bit difficult to explain but nurbs are al about mathematics and a bit intolerant to human errors.