See this reddit post, where the OP has created this web site dedicated to known Google Now commands and their variations. There is no direct way of silencing, so here is a workaround
When Google is reading the results and you heard enough, say OK Google mute the volume
This silences Google by setting volume to zero. To revert to normal volume, say OK Google set the volume to x
, x being a number from 1 to 15 (you will need to experiment to get to comfort level) or OK Google set the volume to full
Tested this on searches like OK Google define aerodynamics, where the rant (as you say ) was long enough . I haven't been able to configure to read out search results, say on Donald Trump :)