Not sending a channel is not a concept that exists in DMX. You always have to send all the channels in the universe, on every update. It's how the protocol works.
The closest you can get is to send 0 on all the channels you want to have control over on the desk, and use HTP merge. This will write over all the 0s if the desk sets them to a higher value.
You can't currently have blank channels within a fixture.
The fixture creator will let you create 'dummy' channels for all parameters in a fixture, and even have them send a default value.