OrCAD PCB 16.6 - copper heat sink for a DFN with no electrical connections

I take it that the epad of the LTC2942-1 is defined as a pin on the schematic symbol.
You can then assign a net to it like "HEAT". Just draw a wire out to nothing.
(There will be a schematic DRC-warning about it ("one net") but it can be ignored.)

Then in the layout editor, simply create
the shape as above and assign it to that net. The shape "HEAT" will then be isolated
from other shapes on that board but of course connected to the epad.

In the PCB Editor, set selection to "PIN", select the epad, then EDIT | Properties and set "DYN_Thermal_Con_Type"
to "Full Contact" This will connect the epad without any thermal relief connects.

Note that this can also be done already when creating the footprint.

Handy also for various mechanical stuff like mount holes, etc.

Thanks Ulfk! Worked perfectly.

But what is the best-practices way of getting rid of the DRC errors (shape too close to pin)? I don't want to narrow it down where the heat sink copper meets the epad. Should I waive the DRC's, or create a Region around the device and change the constraints?

There are (most likely more than) two ways to accomplish this.

1. Select the shape "HEAT", right click on it and change the shape (clearance) parameters
If you check around that menu, you can also set thermal relief to "Full" for SMD
pads. Even if a specific pad has not been given a "Full Contact" property.

A drawback is that you will have to do this manually every time that component is used.

2. Create a region as you suggest above.

But then, if you create the shape manually in detail and not letting it "flow"
all around the LTC2942, then you are in total control of how it behaves.

Usually, I set the "Full Contact" property on the Epad, then create the shape around it
manually if it is tied to another net than GND. If the Epad IS GND and a specific clearance
is required, I use "Region".

Accepting DRC's and waiving them is IMO the lest preferrable method.
Sooner or later, when in a hurry, one DRC too many will be waived...