prop is the correct function to call. However, I suspect that the field you are trying to disable is part of the new content that comes from the load call. If that is the case, then you must only try to access it in the completed callback from the load.
Ah yes, if that's the case then the element won't yet exist when the statement is executed. Though I'd use the success callback rather than the complete callback (which is also called in the event of an error).