Speech Recognition, ContextualStrings

more context : Adding speech recognition to my app, to perform a specific data entry task. There is small list of specific words which are valid in the context of the task.

The .contextualString parameter for the recognition request I believe provides additional words for context.

What I want to do is sort of the opposite - I want to suppress words which are not in the list of expected words. For example, the input is looking for the word four, but it returns fore amd for depending on other words its sees before or after. looking for two, and I could get to or too, again, depending on the sequence. Is there a parameter I could add to limit the words appropriate to return for context, or specify a dictionary? other examples are looking for heart and could get art. Lookinf for spade and sometimes get speed. I can manually try to filter and look for multiple things to represent what I am looking for, but want to be sure this isn't available as a parameter feature I just haven't found.