On-line supervised spotting and classification of subsequences can be performed by comparing some distance between the stream and previously learnt time series. However, learning a few incorrect time series can trigger disproportionately many false alarms. In this paper, we propose a fast technique to prune bad instances away and automatically select appropriate distance thresholds. Our main contribution is to turn the ill-defined spotting problem into a collection of single well-defined binary classification problems, by segmenting the stream and by ranking subsets of instances on those segments very quickly. We further demonstrate our technique's effectiveness on a gesture recognition application.