I want to rule out one of the choices in cdfCreateParam.
I suppose ?use is what is need, but how do I use it?
By "rule out" do you mean "not display"? If so, the ?display callback is probably the best choice - then specify ?display "nil". The ?use setting isn't really used consistently, so I'm not entirely certain whether it's useful or not...
Sorry, I wasn't very clear. It's a radio field I'm creating. I want to rule out one of the options, depending on other parameters. Is this possible?
Not by doing it this way. You can have a callback which updates the choices on the fly though. See Unable to change choices of a cyclic CDF parameter through callback (the article title reflects that it wasn't possible in the past).
However, from a quick check, it seems that you can only change the choices for cyclic fields, not radio fields.
So for a radio field it's not possible at all.