I think we can, for the most part. There was another thread on here where a few people talked about that. I think it was one of the sv threads. It's something about controlling your breathing. Gumdropper seems to know a lot about that, so you might want to PM him with the question, or maybe he will respond to this thread.
I know there have been times where I was ill, and actually was willing to let it happen, and it didn't. I was so n* one particular time any 'normal' person would have v*. I think emets have a lot of control over it, if they choose. I doubt it's foolproof, but along with antiemets, I think we have a pretty good chance!