The initials URC in that model number are not related to the URC brand name that is the topic of this thread.
You are in the wrong forum.
The right forum is:[Link: remotecentral.com]
Also, the Home Theater Key is not a device key. Lots of people are confused by the relationship between HT and a device key and ask questions like the one you asked. But you might get a better answer and/or understand the answer better if you realized HT mode is very different from a device mode. It is another layer on top of the device modes.
Also, there is a default way in which HT mode is layered on top of the device modes, so that clearing out any HT mode customization you have done would not cause key presses in HT mode to do nothing. It would cause key presses in HT mode to be passed through to other modes in a default arrangement.
I'm not sure what you might be able to do without a JP1 cable to get keys in HT mode to do nothing. Do you really want that? Why?