We recommend you experiment with the "reject threshold" setting for the QA in question. This setting is a confidence level below which the engine will not accept as a valid recognized utterance. If it is set too high, then it may give a lot of recognition failures of otherwise good utterances. So, you have to strike a balance.

The parameter is set in Visual Studio .NET. In the design view, right-click the QA in question, select property builder, and then select the input section. On the right hand side, there is a table where you map the semantic item to an SML path. There you will find a column to set the rejection threshold.